REMAINS OF REPTILES AND FISHES. 113 



Pternodiis productus, and Ochlodus crassus, desci'ibed in the paper 

 "On the New Coal Fishlets," are all referable to Dvplodus. The 

 genus Dittodus is established on two very dissimilar fossils : D. 

 parallelus is, we have already seen, founded on the fragment of 

 a jaw with a few of the teeth of Bhizodopsis sauroides ; Dittodus 

 divergens (pi. 2) is apparently nothing more than Dij^lodus mi- 

 mulls'^' of Agassiz ; and, like his figure, that given by Professor 

 Owen is represented without the small central spine : indeed it 

 is scarcely possible to show it in such a section as that figured 

 in pi. 2. The size, form, and histological characters all agree 

 with those of our sections of the minute specimens of Diplodus. 



Pternodus productus (pi. 10) is the single-spined variety of Dip- 

 lodus gibhosus seen in profile, with a well produced base ; or it 

 may possibly be a lateral section of a fully developed specimen 

 in which one of the large spines only is exhibited. In either 

 case the same appearance would be presented of the large prO' 

 jecting "heel," with its outline sweeping into the curve of the 

 spine ; and, in fact, the form, proportions, and size all exactly 

 agree with those of similar sections in our possession of the 

 single-spined variety of Dip)lodus. The minute structure is pre- 

 cisely the same ; the greater portion, however, of the basal mar- 

 ginal boundary, from m to b in fig. 1, pi. 10, has been ground 

 away; and that which is designated " osseous tissue of jaw" is 

 merely a portion of the osteo-dentine of the pulp-cavity. 



There are two species of Aganodus described : one, A. cqncalis 

 (pi. 9), is based apparently on a section made from before back- 

 wards of a single straight spine of the small variety of Diplodus. 

 The two processes (o) below the spine are projecting portion of 

 the base, the most of the base itself having been broken away. 

 The opening between the two processes is in part a natural ca- 

 vity, frequently seen in sections. A. undatus (pi. 10) is a lateral 

 section of a single minute spine of the same variety of Diplodus, 

 somewhat abnormal in form. There is no difference of import- 

 ance in the minute structure, and it exhibits, in a most distinct 

 manner, the numerous concentric layers of dentine mentioned by 

 M. Agassiz as characteristic oi Diplodus (vol. iii., p. 209). 



* ToissoDS Fossiles," vol. iii., )>. 20.'i, T:ib. X.MI., f. 0-8. 



n 



