14 



Fossil Fishes. 



Table-case, 

 No. 33. 



complete specimens from the Permian of Germany, these fishes had 

 a long barbed spine (named Pleuracanthus (Fig. 11?'), Orthacantlins, 

 or Xenacanthus) fixed immediately behind the head ; the teeth were 

 originally described as JJiplodus, from the crown consisting of two 

 main cusps ; and there are distinct interapinous bones supporting 

 the rays of the dorsal fin. Cladodm is also placed near this family. 



Fig. 28. — Jaws of Male (A) and of Female (B) Thornback Skate, Raja clavata, showing the 

 remarkable variation in the dentition which they exhibit. 



Fio. 29.— Dermal Spines of the Thornback, Raja clavata. 



Order III.— CHIMJEEOIDEI or HOLOCEPHALA. 



The Chimaeras resemble the Sharks in many important features, 

 but, in the skull, the upper jaw is fused with the cranial cartilage, 

 not suspended by the upper part of the hyoid arch. The skeleton 

 is wholly cartilaginous, and the notochord is tolerably persistent, 

 the vertebrae being represented by mere slender rings. In the two 

 living genera, there is a strong spine in front of the dorsal fin : the 

 gill-clefts are covered by a fold of skin, so that only a single 

 external opening is observed : and the dentition consists of four 

 plates above and two below. 



Teeth of Rhyncliodus, from the Devonian of Korth America, are 



