VERTEBRAL COLUMN OF FISHES. 57 



ternally a thin layer of hyaline cartilage, internally a thin layer of 

 bone, and, between these, two alternate layers of semi-osseous and 

 hyaline cartilages. 



In the flat Plagiostomes (Skates, Rays, Torpedos) vegetative re- 

 petition manifests itself still more strongly in the multiplication of 

 vertebi'aj, and especially of the central elements ; which, as indicated 

 by their rudimentary primary ossification in Chimcera and Heptan- 

 chus, are commonly more numerous than the more constant neural 

 arches ; nor are interneural and interho3mal pieces altogether wanting 

 in the Rays. JNIiiller (xxi. p. 92.) rightly states that in Kaia cla- 

 vata these ossa intercalaria constitute the chief part of the neural 

 arch, at the anterior part of the vertebral column ; whilst the neura- 

 pophyses resume their ordinary share in its formation at the posterior 

 part of the column. In the Zygcena we perceive, also, interspinal 

 cartilages. In Rhinohatus a single spine answers to two vertebral 

 bodies (xxi. p. 93.), and we may well suppose this multiplication of 

 central pieces to have been carried still farther in the primaeval fossil 

 Ray (ySpinachorhinus) from the Dorsetshire Lias.* 



In the anchylosed cervical vertebrae of the Skate the short cen- 

 trums are indicated by transverse bars along the middle of the under 

 part. The parapophyses in most Rays pass forwards, and are then bent 

 backwards, the angle of one fitting, like an articular process, into the 

 notch of the parapophysis in advance : they do not support pleura- 

 pophyses ; they gradually bend down behind the pelvic arch, and 

 complete the hasmal canal about six vertebras beyond it ; the htemal 

 spines become fiattened in the tail of some Rays. 



In the 'Pisces ossei ' of the Cuvierian system, which include 

 the great majority and typical members of the class, it might be ex- 

 pected that ossification, of the vertebral axis at least, would be a 

 constant condition : yet I have already had occasion to allude to a 

 fish, viz. Lepidosiren, in which the embryonic state of the bodies of 

 the vertebra3, as a continuous chondro-gelatinous chord, remains ; 

 although the neur- and par-apophyses, many cranial bones, and the 

 maxillary, mandibular, hyoidean and scapular arches, are well ossi- 

 fied. The fact of many fossil Ganoid fishes showing the same parts 

 of the skeleton petrified and undisturbed, but without a trace of the 

 central elements of the vertebra?, shows that the transitional condition 

 of the Lepidosiren's skeleton was not uncommon in the primaeval 



* Sqnahnda of Ililey and Stutchliury (Geol. Trans. 2(1 ser. vol. v. p. 83. pi. 4.), 

 regarded as a fossil reptile by Dr. Grant (Lectures, I>ancct, Jan. 1834, p. 576.): 

 170 vertebral l)odies are included in tlie abdominal part of tlie column ; and the 

 part extending beyond the pelvic arch, if equal to that in most Rays, probably did 

 not contain less than four times the above number of abdominal vcrtebr;e. 



