372 Messrs. Hancock & Atthey on Roeptile-‘andl Fish-Remains 
size. The largest are three-quarters of an inch from the base 
to the apex of the large or lateral spines; the smallest, mea- 
sured in the same way, are not more than one-twelfth of an 
inch in extent. Between the two extremes, tubercles of every 
size occur. Now the smaller individuals, which are by far 
the most numerous, agree very well with Déplodus minutus 
of Agassiz, so far as the imperfect specimens described and 
figured by that author permit a comparison. M. Agassiz says 
he was not able to discern the median cone; but this is not 
to be wondered at, for none of his figures represents the base 
entire. 
Note.—Dittodus divergens, Aganodus apicalis, Aganodus 
undatus, Pternodus productus, ior Ochlodus crassus, described 
in the paper “ On the new Coal Fishlets,” are all referable to 
Diplodus. 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 
Rhizodopsis sauroides ; Dittodus divergens (pl. 2) is apparently 
nothing more than Diplodus minutus * of Agassiz; and, like his 
figure, that given by Prof. Owen is represented without the 
small central spine: indeed it is scarcely possible to show 
it in such a section as that figured in plate 2. The size, 
form, and histological characters all agree with those of our 
sections of the minute specimens of Diplodus. 
Pternodus productus (pl. 10) is the single-spined variety of 
Diplodus gibbosus 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 projecting “ 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 Diplodus. The mi- 
nute structure is precisely the same; the greater portion, how-. 
ever, of the basal marginal boundary, from m to 6 in fig. 1, 
pl. 10, has been ground away; and that which is designated 
‘osseous tissue of jaw’ is merely a portion of the osteo-den- 
tine of the pulp-cavity. | 
There are two species of Aganodus described: one, A. api- 
calis (pl.9), is based apparently on a section made from before 
backwards of a single straight spine of the small variety of 
Diplodus. The two processes (0) below the spine are project-. 
ing portions of the base, the most of the base itself having 
been broken away. ‘The opening between the two processes 
* Poissons Fossiles, vol. iii. p. 205, tab. 22. f, 6-8. 
