362 CRETACEOUS FISHES. 
variety of this species, in which the body of the tooth is unusually puckered 
and drawn in at the base; this character is common to all the teeth in the 
specimen, sixty in number. 
Ptychodus decurrens, V.C. (Tab. XXX. figs. 7 & 8; Tab. XXXI. fig. 1; 
Tab. XXXII. fig. 3. Vol. ili. p. 154, Agassiz.) 
This is a common species ; I have only found it in the lower chalk and chalk- 
marl. The principal folds of enamel are straight, small and numerous, extending 
generally to the edge of the tooth, which on this account is easily distinguished. 
The larger specimens are somewhat convex, the smaller ones flat. It has been 
found in France, Italy and Germany: Captain Burgh residing at Brighton has 
a mass containing 120 teeth of this species. 
Ptychodus altior, C. (Tab. XXX. fig. 10. Vol. iii. p. 155, Agassiz.) 
This is a well-defined species ; the great folds of enamel are confined to the 
apex of the tooth: the body of the tooth is smooth; its height is considerable, 
and it narrows towards the summit; the enamel is very thin at its base, and is 
more easily broken than in any other species ; in other respects it resembles 
P. mammillaris. 
This may be considered a rare species ; it occurs in the upper chalk, and has 
been noticed by Agassiz in Germany, France and Italy. 
Ptychodus rugosus, new, R. 4. (Tab. XXXI. fig. 5.) 
The Earl of Enniskillen and Sir Philip Egerton have teeth of a species in their 
cabinets resembling in form those of P. altior, found in Kent, but they are larger 
and broader, and the great folds of enamel at the apex stronger, more rugose and 
crenated, and often worn away, showing the remains of calcigerous tubes beneath; 
the body of the tooth is traversed by small folds of enamel, which distinguish it 
from P. altior ; it is also very different from P. mammillaris. 1 propose for it 
the specific name of rugosus. 
Ptychodus latissimus, R. (Tab. XXX. fig. 1 & 2; Tab. XXXI. fig. 3. 
Vol. ii. p. 157, Agassiz.) 
The folds of enamel in this species are larger and the edges more acute than 
in P. polygyrus, and the shape of the tooth is generally more oblong, though I 
have seen some nearly square; the sides are closely beset with coarse granula- 
