348 Messrs. Hancock & Atthey on Reptile- and Fish-Remains 
the mandibular rami. In Rhizodus, however, there appear to 
be two other plates, as we have already seen: these would lie, 
‘one on each side of the median line, immediately behind the 
anterior plate, which is very nearly as wide as the two others 
put together. 
All these bones, as well as several other fragmentary speci- 
mens, have the surface covered more or less densely with 
strong vermicular sculpture composed of hollows and ridges ; 
the latter in some become tubercular, but in others stream over 
the surface smoothly and regularly, with here and there an 
occasional bifurcation ; in others, again, the vermicular grooves 
are intricately involved, and sometimes they are broken up to 
form circular pits. These are the dominant markings in the 
bones already before us; but the sculpturing on the surface of 
some opercular plates which we also assign to the same large 
fish, and which will presently be described, is somewhat modi- 
fied. In these the vermicular ornament is less developed, and 
the pitted and tubercular predominate, the ridges being rough 
and much broken up. 
It is on account of these peculiar surface-characters that we. 
deem these bones to belong to Rhizodus; but this is not the 
only evidence. On the slab with the anterior jugular plate 
there is a portion of a scale of Rhizodus; and on that with 
the two large jugulars several scales of this fish are found | 
lying in contact with them, If we are right in attributing © 
these scales to Rhizodus, we have in the above facts strong 
corroborative evidence that these bones also belong to it. 
The opercular plates above referred to are four in number : 
three are opercles, one is apparently a preeoperculum. ‘They — 
are all crescentic in form, having their anterior margins well 
hollowed, and both extremities considerably produced. The 
largest operculum is six inches from point to point, and is up- 
wards of two and a half inches wide; the posterior margin is 
a little sinuous, and is bordered with cowiral peal depressed 
lines, probably indicative of growth; the anterior margin is — 
bounded by a wide, smooth, articular surface, which is divided 
from the rest of the operculum by a ridge. The preeoperculum 
is similar in form to the operculum, but it is wider in propor- 
tion to its length, and there is a single groove following the - 
sinuosities of the posterior border; the anterior margin is con- 
cave, with a very narrow articular surface, 
Note.—It is the intention in this and the following notes to 
comment on the value of the various genera and species re- 
cently proposed by Prof. Owen in his paper “ On the Dental 
Characters of Genera and Species, chiefly of Fishes, from the 
