ICHTHYOLOGY. 



319 



Classifica- with a single common passage, having only one external opening. 



tion Rudimentary operculum concealed by the skin. Upper jaw repre- 



Chondrop-.spnted by the vomer, the palatines, and tympanals, rudimentary 

 terygians. on the sides of the snout. Hard bony plates supply the place of 

 V ^ -^_y teeth on the jaws, four above, two below. 8nout projecting, with 

 regular rows of pores; a fleshy club-shaped process with spiny 

 point between the eyes. First dorsal armed by a strong spine 

 placed over the pectorals. Claspers of the males trifid ; eggs large, 

 leathery, with fiat velvety borders. 



Genus I. Chim.era, Cuv. Snout conical. Dorsals contiguous, 

 the second reaching to the point of the filamentous tail ; caudal 

 like that of the Sharks under the end of the tail. 



Genus II. Oallorhynciius, Gronov. Snout ending in a fleshy 

 hatchet-shaped appendage. Second dorsal commencing over the 

 ventrals, and, ending opposite the beginning of the caudal. 



CHONDROPTERYGIANS, 



Or Cartilagino\is fislies, are called by Cuvier the second 

 series of the class, and he considers them to be neither in- 

 ferior nor superior to the Osseous fishes forming the bulk of 

 the first series, for many of the genera come near the Ke|)- 

 tiles in the structure of the acoustic organ, and in the geni- 

 tal apparatus, whilst others have so simple an organization, 

 and such imperfect skeletons, that they can scarcely be 

 classed with confidence as vertebrals. He therefore looks 

 upon them as a parallel series to the ordinary fishes analo- 

 gous to the relationship that exists between the Marsupials 

 and the ordinary Mammals. It is right to observe, how- 

 ever, that the very low organization of some Chondroptery- 

 gians here alluded to has reference to the Cyclostonii 

 placed at the commencement of our table, and their exclu- 

 sion, leaving only the Sturgeons, Sharks, and Rays, jire- 

 sents the organization of the Chondropterygians, as of a 

 higher and more equable general character, by which 

 strength is added to Cuvier's reasoning. Yet there are 

 many embryonic characters persistent in the Chondroptery- 

 gians of which the cartilaginous and confluent vertebra;, as 

 departing most from the indurated endo-skeleton of thehigh- 

 est and newest vertebrals, is the most striking. We have 

 already observed that many of the characters assumed by 

 the embryos of existing fishes in the progress of their de- 

 velopment were the permanent condition of the fossil fishes 

 found in the oldest fish-bearing strata, and (as will be more 

 fully explained in the article on Fossil IciiTnYOLOGY, 

 which follows) the researches of Agassiz have shown that 

 as we ascend from the older to the newer deposits there 

 is a progressive predominance of the endo-skeleton over the 

 exo-skeleton, so that the Placoids of that author, or the 

 Plagiostomous Chondropterygians are few in number in the 

 present day compared to the osseous fishes, and the Gan- 

 oids with tlieir heavy armour are much more sparingly repre- 

 sented still. During the chalk epoch both Ganoids and 

 Placoids were in much greater numbers, and they are the 

 only members of the class of fishes which have been found 

 in the deposits below the chalk. Mr Owen has some most 

 instructive sentences bearing on this point in his remarks 

 on the genesis ofjiits. ihe development of fins, he ob- 

 serves, is at first restricted to the region of the head, as in 

 most of the ancient Cephalaspid fishes — Plerichthi/s and 

 Paniphractus for instance — in which the cephalic fins pro- 

 ject fioni the opercular region bet()re the nuchal plates, and 

 there is no trace of true pectoral, ventral, or of vertical fins. 

 In existing fishes the only thing analogous to these cepha- 

 lic fins is the envelopment of the sides of the skull in the 

 Hays by the fore corners of the pectorals, or in Cephalop- 

 tera and Ceratoptera, by what Miilier calls " cephalic fins," 

 but which he considers to be merely detachments of the 

 pectorals, though connected with them only by integument, 

 and not by a continuous succession of fin-rays. In others 

 of the ancient fossil genera there are two closely approxi- 

 mated dorsals and tv\ o anal fins, and both are situated near 

 the end of the tail, which runs into the upper lobe of an un- 



synimetrical caudal fin. Now, in the embryos of existing Classifiea- 

 osseous fishes these vertical fins are developed from a single ''°° — 

 continuous fold of integument, which is extended round the .'-''""'•''"op- 

 point of the tail from the dorsal to the ventral surface ; a ^^^ 

 condition which may be observed in the tadpoles of Ba- 

 trachians, and which is persistent in the Eel and Lepidosi- 

 ren. The growth of this fold is progressive at certain parts, 

 and checked at others ; and in places where develo)iment is 

 active, the supporting dermal rays make their appearance, 

 and the transformation into dorsal, caudal, and anal fins is 

 thus effected. At first the caudal fin is unequally lobed, 

 and the terminal vertebr;t; extend into tiie upper and longer 

 lobe ; the dorsals and anals are also at first closely approxi- 

 mated to each other and to the caudal fin. M. Agassiz has 

 shown that all these embryonic characters were retained in 

 many of the extinct fishes of the old red sandstone ; and the 

 caudal fin did not pass the heterocercal stage in any fish 

 until after the Jurassic or oolitic epoch. The heterocercal 

 caudals are the predominating and almost exclusive form 

 in the existing Sturgeons, Sharks, and Rays, and in most 

 of these one, or both dorsals when there are two, are situ- 

 ated upon the tail, and not rarely near its extremity. The 

 cutaneous seams without rays and other embryonic condi- 

 tions are also common, as may be learnt by consulting the 

 generic characters in the table. 



Cuvier observes that the jiarts of the skeleton of the 

 Selachians (imder which epithet he comprehends the 

 Sharks and Rays) do not assume the fibrous structure of 

 bone ; their interior remaining always cartilaginous, and 

 the induration of their exterior |)roceeding from the de- 

 position of the earthy matter in a granular state, whence 

 their characteristic dotted surface. It is on this account he 

 infers that their calvaria are not divided by sutures, but are 

 composed of a single shell, modelled, however, in other 

 respects after the general plan of an ordinary fish-skull; so 

 that we can distinguish the same regions, similar depressions 

 and eminences, and the same holes. He gives an analysis 

 of the vertebral cartilage of Seluche maximus, which shows 

 that water alone forms ninety per cent, of the mass, while 

 nearly seven are oil and other animal matters, and little 

 more than three of earthy salts. 



Owen selects S(pialina (the Monk-fish), an intermediate 

 form between the Sharks and Rays, as a good typical ex- 

 ample of the characters of the skull of a Plagiostome. The 

 cranial end of the " chorda dorsalis" and its capsule are con- 

 verted into firm granular cartilage, which, extending for- 

 wards from a prominent median basal-ridge, and also ex- 

 panding laterally, constit\ites the oblong flattened plate or 

 base of the crani\uu. Two convex condyles are formed on 

 the posterior margin of this " occipito-sphenoidal " plate for 

 articulation with the end of the spine. Two notches on the 

 side of the basal cartilage define a prominence representing 

 the sphenoidal arch, wliich sends off a rudimental ptery- 

 goid ; and further forward there is an expansion and groove 

 forming the point of suspension of the palato-maxillary 

 arch. Then the cartilage suddenly contracts, and is con- 

 tinued forwards to form the vomerine anterior base of the 

 cranial cavity. The walls of this cavity are as firm and 

 granular as the base, except at the antero-superior end 

 which is closed by membrane only. The foramen magnum 

 posteriorly gives passage to the spinal chord, and there are 

 perforations for the exit of the cerebral nerves. 



The cranial cavity is of larger size than the brain, and 

 communicates only by nervous and vascular foramina with 

 the acoustic labyrinth, which is buried in the side of the 

 cartilage. In the Chimarre, this insulation of the ear- 

 capsules, denoting a high grade of development, is much 

 less complete. The parts of the cranial cartilage, corre- 

 sponding to the separate bones of the skull of an osseous 

 fish, may be recognised by the foramina through which the 

 several pairs of nerves pass ; but the exterior of the skull 



