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,’ 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 Plewracanthus 
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 Plewracanthus” (Fig. 250)°* 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. 
* For references see Zittel’s Text-Book of Palacontology (Eng. trans. ed. by C. R. 
Eastman), London and New York, ii. 1902, pp. 22-23. 
® See also restoration of Plewracanthus gaudryi from the Coal-Measures of 
Commentry, Allier, France, by C. Brongniart ; Zittel, op. eit. p. 23. 
