1822.] Geology of Snowdon, and the surrounding Country. 423 
GENTLEMEN, 
In conformity with your expressed wish, I forward to you the 
accompanying drawing (Pl. XVII) of the fossils from Snowdon 
and its vicinity ; and shall now proceed to offer you my opinion, 
or rather my conjectures, upon the nature of each one in parti- 
cular, first observing that scarcely any of them possess sufficient 
‘character to enable me to speak with any tolerable degree of 
certainty. Several of them must remain undecided, until more 
perfect specimens can be obtained; because they are destitute 
of those parts from which generic characters are taken. Almost 
all the organic remains to be traced in these specimens appear’ 
to me to be bivalve and principally terebratuloid shells. The 
specimen numbered | and 2 in the drawing appears to be a cast 
of the inside of the deep valve of a Productus,* of which fig. } 
shows the back, and fig. 2, the hinge: it has distinct but rather 
flat ribs, and it is compressed in a direction from the back to the 
hinge. Fig. 3 is a view of another specimen, which I believe 
to be the same species, but which is compressed laterally, so 
that the ribs are much more prominent. Figs. 4 and 5 are two 
views of an entirely detached little cast which is rather concave 
on one side, and convex on the other. I think this may deci- 
dedly be referred to the genus, if indeed it be a distinct genus, 
described under the name of hysterolites, which we are informed 
in the Dictionnaire des Sciences Naturelles are only found in the 
oldest beds. Asa species, it differs from the only one [ had 
before seen in having distinct longitudinal diverging ribs. Fig. 6 
is a representation of a fossil which I thought at first was proba- 
bly the flat valve of a productus ; but judging from its principal 
features, | am now rather disposed to think it may also be an 
hysterolite. 11 is a very flat impression, and it has two sets of 
diverging ribs, only distinct towards the margin, one set smaller 
than the other, and interposed between the larger. The frag- 
ments represented in fig. || appear to me to belong to the same; 
they are compressed in various directions. There is another 
impression upon the same stone as fig. 6. I have numbered it 
7, and I cannot help expressing some doubt about the real nature 
of this impression ; if it be that of a shell, it is certaimly an im- 
pression of the outside of an avicula. Fig. 8 is probably the | 
impression of the outside of the opposite valve of the same 
kind of shell as fig. 6. The fossil represented at fig. 9 is, 
perhaps, the most singular of all; it appears to be a cast 
of the inside of a terebratuloid shell, and like many of them 
it has several strong diverging ribs, most prominent towards the 
* T make use of this name because itis at present generally adopted. J am perfectly 
aware of the impropriety of using a Latin adjective as a generic appellation, and am conse- 
quently happy to learn from a gentleman who has lately taken much trouble in investi- 
gating these fossils, that the use of this appellation will be superseded, and a name 
which has the right of priority, adopted in its stead. In using the term Productus, I 
do not venture an opinion upon the real nature of these fossils. 
* 
