PISCES. 
67 
Each tooth consists of a thick expanded base, bearing a 
divergent pair of conical cusps at its front edge, usually with 
an intermediate minute cusp. The slender spine is armed 
with a double longitudinal row of hook-shaped denticles, and 
is inserted on the back of the top of the head. The internal 
skeleton, which is sufficiently hardened with lime to be well 
preserved, shows that the notochord was not replaced by 
vertebral bodies. 
Order IY.—SELACHII. 
These are the modern sharks and skates, in which the 
cartilaginous supports of the pectoral fins are fused at the 
upper end into three (occasionally two) basal pieces, with no 
branched arrangement, while the pelvic fins are borne on a 
well-developed pelvis. Vertebral bodies are well formed in 
most members of the Order. 
The Palaeozoic representatives of the Selachii are so imper¬ 
fectly known that they cannot yet be satisfactorily classified. 
The Carboniferous teeth named Psammodus and Copodus (Table- 
case 3) are crushing plates suggestive of those of some of the 
largest existing skates. The Carboniferous and Permian 
Petalodontidse ( Janassa , Petalodus, etc.) are better known, but 
still of problematical relationships (Table-case 3). The 
Carboniferous Cochliodontidae (Table-case 3) seem to have 
been allied to the existing Port Jackson shark (Cestracion), 
with dorsal fin-spines, and with crushing teeth which fused 
together into spirals (like Helicoprion, p. 65) instead of falling 
from the mouth when no longer in use. Cochliodus (Fig. 64) 
Fig. 64.—Jaw with teeth of Cochliodus contortus , from the Carboniferous 
Limestone of Armagh; one-half nat. size. (Table-case 3.) 
is a typical example, and dental plates of this and allied 
sharks ( Psephodus , Pcecilodus, Helodus , etc.) are not uncommon 
in the Carboniferous Limestone. Large portions of small 
fishes referable to Helodus are exhibited from the Stafford¬ 
shire Coal Measures (John Ward Collection, Table-case 3). 
f 2 
Table-case 
2 . 
Wall-cases 
2, 3. 
Table-cases 
3-8. 
Table-case 
3. 
