438 FISHES CHAP. 



Each tooth consists of a broad base, supporting a long pointed central 

 cusp and a variable number of similarly shaped but much shorter 

 lateral cusps. The teeth in the various transverse rows from 

 without inwards are closely wedged together by the interlocking 

 or overlapping of their bases. 



Fam. 1. Cladoselachidae. Several species of Cladoselache, 

 varying from 2 to 5 feet in length, have been found in the 

 Cleveland Shale (Upper Devonian) of Ohio. Isolated teeth 

 similar to those of Cladoselache occur in the Lower Carboniferous 

 of Europe, India, and North America, and have been referred to 

 various species of the genus Cladodus, but with one exception 

 nothing more is known of the structure of these Fishes, and 

 consequently their relationship to Cladoselache is doubtful. 

 C. neilsoni, 1 from the Lower Carboniferous (Calciferous Sand- 

 stones) of Kilbride in Scotland, has a very different type of 

 pectoral fin, which appears to be distinctly uniserial, but inter- 

 mediate in structure between the biserial fin of Pleur acanthus 

 and that of the modern sharks. There are several other genera 

 from the Devonian and Lower Carboniferous whose claims to 

 inclusion in this group rest on no better foundation. 



Order II. Ichthyotomi. 



While more specialised than the Pleuropterygii the Fishes 

 included in this group represent an extremely generalised type 

 of Elasmobranch, which, as already indicated, may easily have 

 been the ancestor of more than one group of Fishes. In the 

 typical genus Pleur acanthus* (Fig. 250) 3 the body is elongate, 

 but slightly depressed, with a terminal mouth, and a tapering 

 diphycercal tail fringed above and below by a continuous caudal 

 fin. A long dorsal fin, two small anal fins, and well-developed 

 paired fins with contracted bases, are present. The head is 

 armed with a prominent, serrated, dorsal spine, but it is doubtful 

 if dermal denticles (shagreen) are present. The vertebral column 



1 Traquair, Geol. Mag. (3), v. 1888, p. 81 ; Trans. Geol. Soc. Glasgow, xi. 1897, 

 p. 41. 



' 2 For references see Zittel's Text-Book of Palaeontology (Eng. trans, ed. by C. R. 

 Eastman), London and New York, ii. 1902, pp. 22-23. 



3 See also restoration of Pleuracanthus gaudryi from the Coal - Measures of 

 Commentry, Allier, France, by C. Brongniart ; Zittel, op. cit. p. 23. 



