205 



ICHTHYOSAURUS. 



ICHTHYOSAURUS. 



203 



supporting the doctrine of the transmutation of species, and these 

 animals have been regarded as trausitionary forms from the Fish to the 

 Reptile. Professor Owen thus refers to this opinion, in his ' Report 

 on British Fossil Reptiles," made to the British Association in 1839 

 and 1840 : 



" The nearest approximation to the organisation of fishes is made 

 by the Ichthyosaurus, an extinct genus which appears to have been 

 introduced into the ancient seas subsequent to the deposition of the 

 strata inclosing the remains of the thecodont lizards. The iohthyic 

 characters of this genus of marine Saurians are not of a very important 

 kind, being limited, like modifications of the mammalian type in 

 whales, to a relationship with locomotion in water, while all the 

 modifications of the skeleton which are connected with the respiratory, 

 digestive, or generative functions, are conformable with the highest 

 or Saurian type of reptiles such aa the cranial anatomy (the large 

 size of the intermaxillary bones excepted), the dental structure, which 

 corresponds with that of the posterior teeth in alligators ; the articu- 

 lation of the ueurapophyses to the bodies of the vertebrae; the 

 complicated pectoral arch; the sternum, and complete abdominal 

 cincture of ribs, &c. The circle of numerous imbricated sclerotic 

 bones reaches its maximum of development in the Ichthyosaurus ; but 

 this is an exaggeration of a structure feebly shadowed forth in some 

 existing Saurians, and more strongly shown in birds, rather than a 

 repetition of the simple bony sclerotic cup in fishes. By no known 

 forms of fossil animals can we diminish the wide interval which divides 

 the most sauroid of fishes from an Ichthyosaurus. This most extra- 

 ordinary reptile is a singular compound in which ichthyic, cetacean, 

 and ornithic characters are engrafted upon an essentially Saurian type 

 of structure. The Ichthyosaurus is therefore just such a form of 

 animal aa might be expected, were specific forms unstable, to demon- 

 strate a mutation of characters or some tendency towards a progressive 

 development into a higher and more consistent type of organisation ; 

 nor is the field for testing the transmutation theory less ample than 

 the subject is favourable. We have the opportunity of tracing the 

 Ichthyosauri, generation after generation, through the whole of the 

 immense series of strata which intervene between the new red-sand- 

 stone and the tertiary deposits. Not only however is the generic 

 type strictly adhered to, but the very species which made its first 

 abrupt appearance in the lowest of the oolitic series maintains its 

 characters unchanged and recognisable in the highest of the secondary 

 strata. In the chalk formations, for example, the genus Ichthyosaurus 

 quits the stage of existence as suddenly aa it entered it in the lias, 

 and with every appreciable osteological character unchanged. 



" In the different species of the Ichthyosaurus founded upon minor 

 modifications of the skeleton, several appear contemporaneously in 

 the strata where the genus is first introduced ; and those which 

 remain the longest manifest as little change of specific as of generic 

 characters. There is no evidence whatever that one species has suc- 

 ceeded or been the result of the transmutation of a former species. 

 The tenuirostral Ichthyosaurus existed at the same time, and under 

 the same external influences, aa the stronger and shorter-jawed 

 Ichthyosaurus cammunis ; just as the tenuirostral Dtlphinus Ganyeticus 

 co-exists at present with the short-jawed porpoise. 



" If the relative periods of existence of the different Enaliosauriau 

 reptiles were not well ascertained, and room were allowed for con- 

 jecture as to their (successive appearance on. this planet, it would be 

 as easy aa seductive to speculate on the metamorphoses by which their 

 organic framework, influenced by varying conditions during a lapse 

 of ages, might have been gradually modified, so as to have successively 

 developed itself from an Ichthyosaur to a Plesiosaur, and thence to a 

 Crocodile. 



" We may readily conceive, for example, the fish-like characters of 

 the vertebral column of the Ichthyosaurus to have been obliterated 

 by a filling iip of the intervertebral cavities through ossification of 

 the intermediate elastic tissue, and the Plesiosaurian type of vertebra 

 to be thus acquired. The normal digits of the fin might be supposed 

 to become strengthened and elongated by more frequent reptation 

 on dry land, and thus to cause an atrophy of the supernumerary fingers ; 

 phalanges of a more saurian figure might have been produced by the 

 confluence of a certain number of digital ossicles ; the head might 

 be shortened by a stunted growth of the intermaxillary bone, and 

 thus be reduced to Plesiosaurian proportions. The teeth might become 

 more firmly fixed by the shooting of bony walla acrosa their interspaces, 

 as in the young Crocodiles. 



" If we now elongate the bodies of the vertebra), reduce some 

 twenty pairs of anterior riba to hachet-bones, place the fore paddlea 

 at a corresponding distance from the head, and the hind paddles pro- 

 portionately nearer the tail, little more will be required to complete 

 the transmutation of the Ichthyosaur into the Plesiosaur. If next, 

 in adaptation to a gradual change of surrounding circumstances, the 

 jaws of the Plesiosaur become lengthened to the proportions of those 

 of the Tenuirostral Ichthyosaur, but at the expense of the maxillary 

 'I of the intermaxillary bones, preserving the socketed implanta- 

 tion of the teeth ; if, to balance the elongation of the jawa the neck 

 the same time nhrank to nearly its former Ichthyosaurian propor- 

 tion*, with some slight modification of the Plesiosaurian type of the 

 r;e ; if a further development and a more complete separation 

 of tho digits of the fore and lu'ud members were to take place, so that 



they might serve for creeping as well as swimming ; if the exposure 

 of the surface to two different media, and of the entire animal to 

 perils of land as well as of sea, were to be followed by the ossification 

 of certain parts of the skin, and the acquisition by this change of a 

 dermal armour such we might conceive to be the leading steps in 

 the transmutation of the Piesiosaur into the Teleosaur. 



" And if the three forms of extiiict Saurians, whose changes of 

 specific and generic characters have thus been speculated on, had 

 actually succeeded each other in strata successively superimposed in 

 the order in which they have here been hypothetically derived from 

 one another, some colour of probability might attach itself to this 

 hypothesis, and there would be ground for searching more closely into 

 the anatomical and physiological possibilities of such transmutations. 

 Ichthyosaurus, Plesiosaurus, and Teleosawus are however genera which 

 appear contemporaneously on the stage of vital existence : one neither 

 preceded nor came after the other. How the transmutation theory is to 

 be reconciled to these facts is not obvious ; nor to these other, viz., that 

 the Teleosaur ceases with oolite, while the Ichthyosaur and Plesiosaur 

 continue to coexist to the deposition of the chalk, and disappear 

 together alike unchanged ; the Ichthyosaur manifesting as little ten- 

 dency to develop itself into a Plesiosaur, as this to degrade itself into 

 the more fish-like modification of the Enaliosauviau type." 



One of the finest collections of the remains of these animals existing 

 is that in the British Museum. Altogether " there are about thirty 

 very fine specimens, including the most interesting of the separate 

 crania, paddles, vertebral columns, &c., besides a great number of 

 isolated bones, parts of skulls and jaws, coprolites, and other remains 

 of Ichthyosauri." (Mantell.) 



The following are the species of this genus which have been at 

 present made out : 



/. tenuirostris (Conybeare). It is characterised by the great length 

 and slenderuess of the jaws, which resemble in this respect the 

 maxillary organs of the Gavial, or Teleosaurus. The length of the 

 mouth is produced by the elongation of the intermaxillary bones and 

 of the deutary bones of the lower jaw. The malar bone is remark- 

 ably long and slender. The cranium is flat, and the orbits are very 

 large. The teeth slender, 60 to 90 on each side the Upper jaw, and 

 60 on each ramus of the lower jaw. The anterior extremities, or 

 paddles, are much larger than the posterior pair, and very strong and 

 massive. The shafts of the humerus and femur are relatively long, 

 and their distal ends broad. This species attains a length of 1 3 or 

 14 feet; the largest teeth are an inch and a half in length. It was 

 named by Mr. Hawkins I. chirostrongulostinus (Round-Boned-Paddle 

 Fish-Lizard). It is figured in his remarkable work on these creatures. 



/. intcrmedius (Conybeare). The teeth are more acutely conical 

 than in /. communis, and the stria: less prominent, but are less slender 

 than in the preceding species. The maxillary portion of the skull is 

 relatively shorter, and converges more regularly to the front than in 

 /. communis, and the teeth are longer, more slender, and numerous. 

 This species has been the most commonly found, and ranges through- 

 out the Lias formation of England. There are two specimens in the 

 British Museum, one 7 feet in length, the other 6 feet. 



/. longipennii. The species thus named in the British Museum is 

 from the Lias of Whitby in Yorkshire. Dr. Mantell says that he is 

 not aware of any description of this species. 



/. communis (Conybeare). The skull is wide behind, and rapidly 

 contracts to the base of the jaws, which are prolonged and sub-com- 

 pressed. The teeth are relatively large, round, conical, and longi- 

 tudinally furrowed, the base being expanded and deeply grooved, 

 40 to 50 on each side the upper jaw, and 25 to 30 in each ramus of the 

 lower jaw. The anterior paddles are three times longer than the 

 posterior pah-, and are proportionately broader, and contain a greater 

 number of ossicles than in other species. This character induced 

 Mr. Hawkins to call it /. chiropolyostinus (Many-Boned-Paddle Fish- 

 Lizard). There are 8 metacarpal bones and 212 ossicles. The 

 humerus is relatively shorter and stouter than in any other species. 

 It has 17 sclerotic plates to the eye. This is one of the larger species 

 of the genus, as some of the specimens indicate a length of 14 feet. 

 It is supposed this species had a large tegumentary caudal fin, which 

 is rendered probable by the laterally compressed form of the terminal 

 caudal vertebrae, as first ascertained by Sir Philip Egerton. 



/. platyodon (Conybeare). This is the largest of the species, and 

 specimens have been procured 30 feet in length. The orbit iu some 

 specimens is 1 foot in diameter, and the teeth 2J inches in length. 

 The teeth are distinguished by the form of the crown, which is 

 conical and subcompressed, and has a sharp ridge on each side ; the 

 base is not grooved as in the teeth of /. communis. This character 

 of the teeth suggested the specific name. The dental formula is 



4545 



The vertebnc are about 120 in number. The anterior and 



4040 



posterior paddles are equal in size. This structure induced Mr. 

 Hawkins to name this species /. chiroligostinus (Few-Boned-Paddlo 

 Fish-Lizard). Tho humerus is short in proportion to its breadth. 

 The femur ia proportionately larger than in other species. 



/. lonchiodon (Owen). The teeth are more slender in proportion to 

 their length than in the other species. Their base is cylindrical and 

 regularly fluted ; the transverse section of the crown is nearly circular, 

 not compressed. The paddle is relatively lass than in the last species. 



