8 4 



PLEURACANTHID SHARKS 



Fig. 90 A. Teeth of Pleuracanthus. 

 (After DAVIS.) 



XJ. 



essential to sharks. That it was actually a shark cannot 

 be doubted ; its gills, six or seven in number, opened 

 separately to the surface ; its teeth (Fig. 90 A) were 

 typically shark-like, arranged in many rows on Meckelian 

 and palatoquadrate cartilages ; a tuberculated dorsal spine 

 was present ; claspers occurred in the male ; the vertebral 

 column, although notochordal, N, presented intercalary 



plates, 1C, and the 

 jaw was essentially 

 hyostylic, HM. On 

 the other hand, 

 many of its struct- 

 ures are clearly tran- 

 sitional to the Dip- 

 noan: the pelvic fins 

 are shark-like, with 

 the radial supports, 

 R, arising from but 

 one side of the line 

 of basals, B ; but the 

 pectoral fin is typi- 

 cally archipterygial, 

 and the caudal diphy- 

 cercal, as in the lung- 

 fishes. In this re- 

 gard the continuous 

 dorsal fin, with its separate basals and radials, B and R, is 

 again noteworthy. But most singular of all the features 

 of this lung-fish-like shark were its integumentary charac- 

 ters ; shagreen tubercles had disappeared on the body sur- 

 face, and derm bones had appeared roofing the head : their 

 arrangement (Fig. 90 B) is strikingly similar to that of the 

 lung-fish of Fig. 124. 



Fig. 90 B. Dermal bones of the head roof of 

 Pleuracanthus. X 5. (After DAVIS.) 



