201 



ICHTHYOSAURUS. 



ICHTHYOSAURUS. 



203 



third, by the union of their 

 bases, produce a triangular 

 socket on the under surface 

 of the atlas and axis ; aud a 

 second smaller socket is formed 

 between the axis and the third 

 vertebra by a similar dispo- 

 sition of the fourth and fifth 

 planes." Sir Philip adds that 

 the second bone of the series 

 is frequently found with the 

 atlas and axis, and is not un- 

 commonly fixed in its position 

 by anchylosis. The third bone 

 he states to be of rare occur- 

 rence, in consequence of its 

 diminutive size, and he thinks 

 that in some species it is pro- 

 bably altogether wanting. He 

 designates these bones as Sub- 

 vertebral Wedge-Bones. The 

 reader will find in the same 

 interesting memoir many valu- 

 able observations on the struc- 

 ture and articulation of the 

 cervical vertebra?, the com- 

 bined result of which, and of 

 the reduction of the inter- 

 vertebral cavities, must, as Sir 

 Philip remarks, have been a 

 considerable increase of power 

 in this part of the spinal 

 column ; and he further states 

 that, proceeding from the 

 lumbar vertebras towards the 

 head, the column attains its 



Head and Sclerotic Plates of Ichthyosaurus. 



minimum diameter about the fifth cervical vertebra, from which point 

 to the occiput it increases in size very rapidly. 



The ribs appear to be constructed more upon the eauroid type, 

 for they are continuous along the vertebral column from the head to 



followed by the bonea of the 

 fore arm ; and these are suc- 

 ceeded by numerous regularly- 

 disposed polygonal bones, ex- 

 ceeding, in some species, the 

 number of one hundred, which 

 form the paddle or fin. In 

 form these bones differ both 

 from the phalanges of lizards 

 and whales. 



The bones of the pelvis 

 closely resemble those of the 

 crocodile, and, as Sir H. De la 

 Beche aud Mr. Conybeare ob- 

 serve, the femoral bone aud 

 posterior paddle are altogether 

 analogous to the hutnerus and 

 anterior paddle ; but, contrary 

 to the development of the 

 posterior extremities of quad- 

 rupeds in general, they aie 

 very considerably smaller, 

 nearly iu the proportion of 

 one to two. 



Dr. Buckland, with refer- 

 ence to the posterior extremi- 

 ties, or paddles, observes, that 

 these are wanting in the Ceta- 

 ceans, and that they possibly 

 compensate for the absence of 

 the flat horizontal tail with 

 which those animals are fur- 

 nished. In a paper, ' On the 

 Dislocation of the Tail, at a 

 certain Point, in the Skele- 

 tons of many Ichthyosauri,' 



Lower jaw of IMhyoiaurai. Dr. Buckland. 



the pelvis ; they are slender and mostly bifurcated at the end, and 



many of them are united in front across the chest. Intermediate 



bones, analogous to the sternal aud intermediate costal cartilages iu 



the crocodiles and the sterno- 



coetal arcs in Pluiotauna, 



united the ribs of the right 



side to those of the left. Dr. 



Buckland is of opinion that 



this structure was probably 



subservient to the purpose of 



introducing into their bodies 



an unusual quantity of air, 



the animal being by these 



means enabled to remain long 



beneath the water without 



riding to tbe surface for the 



purpose of breathing. 



In the sternum we find a 

 combination of bones admir- 

 ably adapted for resistance. 

 Of this part of the bony frame- 

 work Mr. Conybeare says, 

 " The form of the sternal arch 

 and the broad surfaces of the 

 clavicles is such as to impart 



great strength to the chest, SUraal arch and nntcrlor dd , M of TM , ;,,.,. 



enabling the animal to breast 



the most disturbed waters, and affording an extensive surface for 

 the attachment of powerful muscles to assist in moving the ante- 

 rior extremities ; " and Dr. Buckland remarks that the bones com- 

 posing this arch are combined nearly in the same manner as in the 

 Ornithorhynchui of Australia, which geeks its food at the bottom of 

 lakes and rivers, and is obliged, like the Ichthyiitaunu, to be con- 

 tinually rising to the surface to breathe air. To this sternal arch the 

 anterior paddles are articulated ; they are nearly one-half larger than 

 the posterior paddles, and in this part of the structure the cetaceous 

 type appears to have been followed. The short and stout humerua is 



read by Professor Owen, before the Geological Society of London, the 

 author commenced his observations by referring to the skeleton of 

 the existing Cetacea, and pointing out how slight is the indica- 

 tion afforded by the caudal vertebra) of the large terminal fin, 

 which forms in that class so important an orgau of locomotion ; and 

 the improbability that its presence would have been suspected had 

 the Cetacea been known only by their fossil remains, in consequence 

 of the fin having consisted entirely of decomposable and unossified 

 material. 



He stated that the depressed flattened shape of the terminal 

 vertebra), which gives the only indication of the horizontal flu (and 

 which character is not present in all the Cetacea), is not recognisable 

 in the skeletons of the Ichthyosauri and Plesiosauri ; but he proceeds 

 to describe a condition of the tail in the skeletons of the Ichthyosauri 

 which, he conceives, affords an indication of a structure in the 

 extinct animal analogous to the teguruentary fin of the Cetacea, and 

 which has not been suspected by the authors of the conjecturally- 

 restored figures of the Ichthyosauri already published. The condition 



alluded to is described as an 

 abrupt bend of the tail, abouo 

 one-third of its whole length 

 distant from the end, and at 

 the 30th caudal vertebra in 

 the Ichthyosaurus communis ; 

 the broken portion continuing 

 beyond the dislocation as 

 straight as in the part which 

 precedes it. As there is no 

 appearance of a modification 

 of structure in the dislocated 

 vertebra;, indicative of the tail 

 having possessed more mo- 

 bility at that point than at any 

 other, and as the dislocation 

 has taken place at the same 

 point in seven specimens 

 examined by the author, he 

 conceives that it must be 

 due to some cause operating 

 in a peculiar manner on 

 tbe dead carcass of the Ich- 

 thyosaurus, in consequence of 



some peculiarity of external form, while it floated on the surface of 

 the sea. 



A broad tegumentary fin composed of dense but decomposable 

 material, he observed, might have been attached to the terminal 

 portion of the tail; and such a fin either by its weight, or by 

 presenting an extended surface to the beating of the waves, or by 

 attracting predatory animals of strength sufficient to tug at, without 

 tearing it off, would occasion, when decomposition of the connecting 



ligaments had sufficiently far advanced, a dislocation of the vertebra) 

 immediately proximate to its point of attachment. The two portions 



