ANATOMY OF VERTEBRATES. 71 



huge terrestrial Dinosaurs, and other extinct grouiis with the same 

 costal structure. The existing lieptiUa are but a remnant of a 

 once extensive and varied class of cold-blooded vertebrates, which, 

 since the mesozoic epoch has been on the wane.' 



§ 26. Developement of the skull. — In reviewing the modifications 

 of this part of the vertebral column in the Ilannatocrya, we 

 retrace our steps to the lowest water-!n-eathing forms, and 

 recommence with the Dermopterous subclass. 



Passing from the trunk to the head, we find in the Lancelet 

 {-f>r(i)ichi.osto)na^, fig. 23, that the cranium is not indicated by 

 ditt'erence of size or structiu-e of the rudimental vertebral cohnnn, 

 but consists of the gradually contracting anterior termination of 

 tlie neural canal, which retains its primitive fibro-membranous wall, 

 ?!, ob, without any superaddition of parts, and is supported l)y the 

 tapering end of the iiotocliord, ib. eli. This part extends farther 

 forward than the cranial end of the neural canal, indicating the 

 non-developcment of the prosencephalon and corresponding part 

 of the cranial cavity. In fact, there is no ganglionic cerebral 

 exf)ansion whatever in this vermiform fish : tlie epencephalon or 

 medulla oblongata is indicated by the origin of the trigeminal 

 nerve, ib. oh, in advance of which the mesencejAahc segment sends 

 oft" the short optic nerve to the dark ocellus, op, and there terminates, 

 somewhat obtusely, beneath what Dr. KiJLLiKER^ has described 

 as a ciliated olfactory capsule, ib. ol. The cranium of the Lancelet, 

 therefore, may be said to be composed of the notochord aud its 

 mcmljranous capsule, without the superaddition of cartilaginous 

 or osseous co\'erings. But, as an appendage to the skidl, may be 

 described the jointed, cartilaginous, hremal arch, ib. h, which 

 extends from below the cranial end of the chorda dorsalis, down- 

 ward and backward on each side of the orifice of the pharynx ; 

 this represents the labial arch of higher INIyxinoids, and supports 

 several pairs of the jointed slender oral filaments. It is tlie sole 

 chondrified part of tlie skeleton in the Branchiostorna. 



The cartilaginous tissue is superinduced upon the fibrous l^rain- 

 sac in osseous fishes, in the following manner. The notochord 

 advances as far as the pituitary sac, or ' hypophysis cerebri,' where 

 it terminates in a point ; cartilage is develojicd on each side, 

 forming a thick ' occipito-sphenoidal ' ^ mass, wliich extends out- 

 ward, and forms the earball or acoustic capsule. The cartilage 

 rises a little way upon the lateral walls of the cranium, and is 

 there insensibly lost in the primitive cranial membrane. At the 



' CLxxx. p. 320. - XXXII. p. 32. 



^ Plaque ntichale, Vogt ; Knucherne basis cranii, Miiller, xxi. 



