CRETACEOUS FISHES. 361 
Notes on the Order Placoidet. 
Family CestracionTID&. 
Genus Ptychodus', Agassiz. 
The genus Ptychodus, formed by Agassiz, is very abundant in the chalk ; he 
considers it more nearly allied to the Cestraciontide than to the Raude. The 
teeth are beautifully preserved; the calcigerous tubes have the same arrange- 
ment as in Acrodus, but the medullary tubes are smaller and less straight. Pro- 
fessor Owen observes in his ‘ Odontography,’ page 61, ‘‘ Ptychodus differs from 
Acrodus in the greater number and more parallel course of the medullary canals, 
their fewer branches, and in the absence of an external layer of finer parallel 
tubes.” The teeth of this genus are sometimes found in large groups. Mr. Catt 
had a mass containing 213 teeth of different sizes, and I have seen others with 
150 or more. The engraved specimen, in sulphuret of iron from Mr. Catt’s 
’ 
collection, of Ptychodus decurrens, is the only instance I have seen in which the 
teeth are preserved in their natural position®. The recent genus Rhina shows on 
a small scale the probable arrangement of the teeth of this gigantic extinct 
genus. The largest tooth in my cabinet measures 34 inches long by 2+ broad, 
and the smallest not more than { of an inch; both are from the same individual 
of Ptychodus latissimus. 
The teeth vary much in shape according to the period of growth and position 
in the jaw, consequently the species are difficult to define. Professor Agassiz 
told me he had only seen one or two large groups of these teeth in all his ex- 
perience. The following species have been found in Kent and Sussex. 
Ptychodus mammillaris, V.C. (Tab. XXX. fig. 6; Tab. XXXI. fig. 4. 
Vol. ii. p. 151, Agassiz, Poissons Fossiles.) 
This species is more common in the upper chalk formation than in the lower ; 
the great folds of enamel do not extend to the edge of the tooth; the teeth 
look puckered, and often resemble a nipple ; the anterior surface is sloping, the 
posterior almost perpendicular: I have a specimen of this species in the centre 
of a flint, which is rare. This species is found in England, France and Germany ; 
Mr. Bass of Brighton has a mass containing 160 teeth. Mr. Catt has a singular 
' arvxy plicatura, occvs dens. * Tab. XXXII. fig. 3. 
