150 ORDEES OF FISHES. 



to sliuffle along the sandy bottom of tlie sea, if stranded at 

 low water. All the species of Pierichthys are exclusively 

 confined to the Old Eed Sandstone. If, howeyev, Astcrole^ns 

 of Pander be identical with Pterichthys, then the genus was 

 represented in tlie Upper Silurian. 



"We may also place in this neighbourhood the gigantic 

 fishes of the Devonian of North America, for the reception 

 of which Newberry has founded the genus Dinichthys. The 

 head in this genus, sometimes measuring not less than three 

 feet in length, was covered by a buckler of ganoid plates, 

 resembling in form and arrangement the plates of the head- 

 shield of Coccosteus, but having their surface adorned with 

 granulations and furrows, in place of the stellate tubercles 

 , of the latter. The most remarkable feature in the genus, 

 however, is the dentition, which closely resembles that of 

 Zcpidosiren, except in point of size, and which must have 

 been associated with carnivorous and predaceous habits. 

 Thus in the lower jaw (fig. 516) the extremity of each 



Fig. 516. — Diagram of the jaws and teeth ot Biniclithys Hcrtzeri, viewed from the front, and 

 greatly reduced in size. From the Devonian of North America. (After Newberry.) 



ramus is bent upwards and pointed, so as to form a huge 

 and sharp tooth on each side ; the margins of the mandible 

 behind these being enamelled for some distance, forming a 

 sharp cutting edge, which may be entire or serrated. In the 

 upper jaw, the tooth-like ends of the mandibular rami are 

 confronted by two great i3ra;maxillary teeth, of a triangular 

 form, and the margins of the maxillae on either side of these 

 are either compressed and trenchant or actually denticulate. 



SuB-OEDEK G. Ohondrosteid^ (Sturionid.^). — In this sub- 

 order the skeleton is almost altogether cartilaginous, and the 

 notochord is persistent. The exoskeleton is usually in tlie 

 form of large ganoid plates, which are united into a shield 

 over the head, but are detached over the body. Sometimes 



