144 



CHIM^EROID PISHES AND THEIR DEVELOPMENT. 



was provided with a well-marked dorsal fin which was supported anteriorly by a 

 spine. This fin, it may be remarked, is the earliest dorsal known in Chimseroids, 

 and its structure, therefore, deserves more than passing mention. Thus, as shown 

 in fig. 140, and in the series of figures, figs. 143 A, B, c, D, its position is further 

 hindward than in recent forms, in this regard suggesting interestingly the condition 

 of shark. It is also noteworthy that the base of the myriacanthid spine is not 

 articulated to the fused mass of anterior epichordalia, but is still connected with a 

 hinder independent plate, b, which, we suggest, becomes in recent Chimseroids the 

 articular process of the anterior cartilaginous plate. A further correspondence with 

 a shark-like condition is noticed in the separation of the fin basis into proximal 

 ("basal") and distal ("radial") moieties; in recent Chimaeroids these are repre- 

 sented by but a single plate, c, 



It should be finally observed that the vertebral column of Myriacanthus, fig. 

 143 B, shows anteriorly a segmentation which reasonably indicates the presence 

 of centra. 



c - s 



Fig. 140. Head region of the Jurassic Chimaeroid Myriacanlhus. After Egerton's specimen, in British Museum. 



C, Centra; S, Detached ventro- median chisel-ihaped "tooth." 



Summarizing our knowledge of Myriacanthus, we note that its dermal defenses 

 are far more highly specialized than in Squaloraja, and that it has evolved an addi- 

 tional pair of tritoral plates in the upper jaw, as well as a ventro-median element 

 in the mandible. Furthermore, that its frontal clasping organ, although still spine- 

 shaped, is less like a spine than in Squaloraja (cf. figs. 131, 132, and 133). On the 

 other hand, in its dorsal fin and in its fairly evident vertebrae it is more distinctly 

 shark-like than any other Chimseroid. 



