32 PISCES. 



seem to persist even to the end of the upturned axis, directly supporting 

 the thick radial cartilages (R) of the superior lobe of the caudal fin. The 

 dermal membrane (D) of the inferior lobe of the caudal fin is supported 

 by simple cartilaginous rays (R) 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 enamelled. 

 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-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. Cladoselache kepleri, C.-fyleri (fig. 24) and other 

 species occur in the Cleveland Shale (Upper Devonian or Lower Carboni- 

 ferous) of Ohio. The largest known examples measure nearly two metres 

 in length. 



ORDER 2. ICHTHYOTOMI. 



Another well-known primitive type of Elasmobranch 

 ranging from the Lower Carboniferous to the Lower Permian 

 and perhaps surviving even later, is that exemplified by Pleur- 

 acanthus. It is a form of fish which might with very little 

 modification become either a Selachian, Dipnoan, or Crosso- 

 pterygian. As in the Pleuropterygii, the endoskeletal cartilages 

 are permeated with minute granular calcifications; and some of 

 the best-preserved crania (Didymodus from the Permian of 

 Texas) exhibit a curious, symmetrical fissuring (hence the 

 ordinal name ICHTHYOTOMI), though there are no membrane- 

 bones. The notochord is rarely or never constricted, but slight 

 calcifications occur in its sheath. The neural and haemal 

 arches are long and slender, rarely with traces of intercalary 

 cartilages. The paired fins exhibit a long segmented axis of 

 cartilage, fringed on one or both sides with other cartilages; and 

 the pelvic fins in the male are produced into a pair of claspers. 

 The right and left halves of the pectoral arch are separate ; 

 and there is no pelvic arch, each pelvic fin being supported 

 merely by its own enlarged basipterygium. The median fins 

 are extensive, supported by slender endoskeletal cartilages 

 which are directly connected and correlated in number with the 



