416 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
The dentition of the genera Cochliodus and Streblodus appears to have been very 
similar, consisting ofa series of three deeply inrolled teeth on the ramus of each jaw. 
The jaws were cartilaginous though it is probable that the cartilaginous substance 
was to some extent permeated by minute grains of ossified matter, similar to the 
chondroid bony structure of some of the plagiostomous fishes. The specimens 
described by Prof. Owen, already referred to, and those to be described in further 
pages of this work, do not indicate that other teeth occupied the intermediate space 
between the two lateral series covering the rami of the jaws. Messrs. Newberry 
and Worthen have, however, described specimens from the Keokuk limestone of 
Illinois (“ Paleeont. Illinois,” Vol. IT., p. 89.) from which it appears that the fish had, 
in addition to convoluted teeth of similar form to Cochliodus, a number of smaller 
teeth resembling, if not identical with, those of the genus Helodus. 
The dentition of Psephodus magnus, Ag., differs considerably from that of Cochliodus 
or Streblodus. The teeth are much flatter and exhibit a very slight curvature, 
especially in the larger teeth. Combined with great variety in outline and form, 
there is a general similarity in the superficial appearance of the teeth as well as in 
the denticulation of the margins. The examination of the magnificent suite of 
specimens in Lord Enniskillen’s collection has suggested the following deductions as 
to the arrangement of the dentition of this genus. 
Many of the teeth are smoothly concave on the under surface, presenting the 
appearance reproduced in Pl. LV., fig. 1; the concavity is in the direction of the 
line of growth, expanding in the usual manner from a comparatively narrow base 
to a widely expanded opposite margin. In Psephodus it will be observed that the 
teeth appear to have increased in size along the margin which enveloped the outer 
side of the jaw. The posterior edge of the largest tooth of the set is more or less 
rounded ; the anterior edge is comparatively straight and fits to the second or 
-median tooth. The concavity or channel is wider at the rounded extremity of the 
tooth and gradually diminishes towards the straight anterior margin. The next 
or median tooth in the series exhibits similar characters, the channel still continues 
to diminish in diameter, being at the widest part equal to the narrowest of the 
previous tooth, and the convolution extending from the inside to the outside of the 
jaw. The third tooth was situated between the median one and the symphysis of 
the two rami of the jaws, it converges rapidly in diameter towards the symphysis, 
and the channel on the underside becomes attenuated almost to a point. In front of 
this there is a long narrow tooth (Pl. LV., fig. 6.) whose surface is raised in the form 
of several obtusely pointed cones. There is no indication that similar narrow teeth 
were attached to either the median or large posterior teeth and it seems probable 
that they were attached in front of the anterior pair of teeth, which are comparatively 
narrow, for the purpose of seizing the objects on which the fishes fed with greater 
firmness and security. In addition to the long and narrow tooth on the external 
margin of the anterior one, there were others which were ranged from its internal 
