ClIONDKiiSTKOUS FISllKS. 



1043 



PISCES CHONDROSTEI 



(STURGEON-FISHES). 



Fishes with endoskeleton principally cartilaginous, but with dermal ossifications repre- 

 senting several of the internal bones of the Teleosts. with shoulder-girdle suspended from 

 the head, with maxillary and palatine arches free from the skull or united therewith 

 merely by a mobile connexion, with branchial cavity, which is situated under the skull 

 and common to the free branchial arches, more or less entirely covered by the opercula, 

 with fully heterocercal caudal fin, and with paired fins unilaterally rayed. Nostrils lateral, 



set just in front of the orbits. 



We liave In-iefiy mentioned above (|). 1) tlie most 

 important anatomical points wherein the Teleosts dift'ev 

 tVom other piscine orders, and we have now to examine 

 the relations obtaining l>etAveen tliose typical fishes 

 which in general organization are by no means inferior 

 to the Teleosts, but which never attain so high a degree 

 of skeletal differentiation or ossification as the latter, 

 and have therefore borne the general name of carti- 

 laginous fishes. As a rule, though with important ex- 

 ceptions, we find that in these forms the ossification 

 deficient in tiie endoskeleton is compensated in the 

 dermal system, which is strengthened or protected with 

 hard, thick (Ganoid) scales, spiny plates, or scutes. In 

 Artedi" they were included, together with the Ci/clo- 

 stomata, in a single order, ChondroptenjgU, which was 

 retained under the name of a series by Cuviek', in 

 contradistinction to the "true fishes" (Teleosts). Agassiz 

 divided this order into two: the Ganoidn", among 

 which he further ranged, on account of their hard 

 dermal gi'owths, several Teleosts (the Glanomorphs, 

 Plectognates, and Lophobranchs), and the Placoidei'', 



among which he placed the Cyclostomc fishes too. 

 Bonaparte' introduced the first reform, and removed 

 the Cyclostomata to a separate subclass, Marsi pohranchii , 

 as opposed to the subclass of the Chinupras, Sharks, 

 and Rays, Elasmobranchii, and Miller-^ eliminated 

 from the order of the Ganoids the said Teleosts. The 

 delimitation of the Ganoids from the Teleosts, however, 

 was no longer based exclusively either on the enamelled 

 scales of the I'oi-mer or on the less advanced calcifica- 

 tion (ossification) of their skeleton, but mainly on the 

 characters adduced above (p. 1) and derived from the 

 more intimate fusion (chiasma) of the optic nerves after 

 their emergence from the brain, and the prolongation 

 of the heart, at the transition to the connuon branchial 

 arterv', into a muscular conus arteriosus, furnished in- 

 ternally with several rows of valves. Thus defined the 

 Ganoids still comprised, in addition to the Chondrostei, 

 the multitude of fishes, possessing more complete jaw- 

 bones, but now for the most part extinct, in which the 

 piscine type has evolved by manifold ])rocesses a wealth 

 of forms rivalling that of tbe Teleosts, but of which 



" Gen. Pise, p. 64. 



' Regn. Anim., etl. 1, torn. II. p. Ill: t-d. ■>. to 



<^ Eech. Poi.w. Foss.. torn. II. 



•* ,, ,, ,, foin. III. 



' Selachiorum tabula anali/tica, Roina? 1839. 



.'' Abh. Akad. Wiss. Berl. 1844, p. 147. 



II. p. 128. 



