98 IOWA GEOLOGICAL SURVEY" 



mouth is also terminal, the upper and lower jaw being similar in 

 shape and size, and supported by a slender elongated hyoman- 

 dibular. The teeth are largest, longest, and most acutely pointed 

 at the symphysis of the jaw, smallest and shortest at the angle of 

 the mouth. The transverse series are closely apposed, and not 

 separated as in the modern Chlamydoselachus ; they are indeed 

 tightly wedged together, and the cusps are frequently much 

 abraded by wear. Every tooth has a principal cusp with variable 

 smaller lateral cusps, and the broad base of each is overlapped 

 by its successor behind. The number of gill-arches is uncertain, 

 but five are known, and the presence of one or two others is 

 suggested by some specimens. The neural and haemal arches 

 of the axial skeleton have been observed only in the caudal re- 

 gion, where they are short tapering rods of cartilage, bifurcated 

 at the base and distinctly corresponding in number with the cal- 

 cified remains of the muscle plates. Intercalary cartilages are 

 wanting. 



The small basal cartilages of the paired fins seem to be em- 

 bedded in the body wall, and the unjointed radial cartilages ex- 

 tend directly outwards to the edge of the membrane. There are 

 no claspers in the pelvic fins, and both these and the pectoral fins 

 were probably mere balancers directed somewhat downwards. 

 Two low dorsal fins have been observed, both destitute of an 

 anterior spine,* but strengthened by simple cartilaginous rays 

 extending to the edge of the fin membrane. No anal fin has been 

 distinguished. The caudal fin is strongly heterocercal and very 

 remarkable. The neural arches seem to persist even to the end 

 of the upturned axis, directly supporting the thick radial carti- 

 lages of the superior lobe of the caudal fin. The dermal mem- 

 brane of the inferior lobe of the caudal fin is supported by simple 

 cartilaginous rays only in its lower portion where they extend 

 quite to the margin. The eye is surrounded by a double series 

 of small dermal plates ; but the remainder of the fish is covered 

 only with minute lozenge-shaped denticles, which are apparently 

 not enameled. The latter are slightly enlarged at the angles of 

 the mouth, where they approximate in size and shape to the 

 smallest of the teeth. The lateral line extends along the trunk 

 between two series of the shagreen-like granules, and was thus 

 presumably an open canal. A short dermal expansion forms 

 a horizontal keel on each side of the caudal lobe just in advance 

 of its upturned end. The largest known examples measure 

 nearlv 2 meters in length." 



*This statement requires to be amended, Dr. Dean having discovered at least 

 one specimen in which the anterior dorsal tin is provided with a powerful Ctena- 

 canthus-like spine. 



