Pvn H,i I'UitlS

ARCA. 175
of sufficient information on this point, it is at present
impossible to make a complete comparison between the
so-called species. All the fossil specimens which I have
seen from the Christiania and Uddevalla districts be-
long to A. glacialis, and they significantly indicate the
climatal conditions which prevailed during the period
immediately preceding the elevation of these sea-beds.
A. pectunculoides, being found in the Coralline Crag,
as well as in the upper tertiaries of Belgium and Sicily,
would appear to be the older of the two. Although I
am not aware of any intervening form having been dis-
covered, such may have existed ;
and supposing that to
be the case, it would be fair to infer that A. pectunculoides
was the ancestor of A. glacialis. Naturalists
have been so much accustomed to regard species in an
objective point of view, and not as abstract ideas, that
it is difficult to bring their minds into the proper frame
of thought for discussing speculative theories upon con-
fessedly so difficult a question as the origin of species.
The present species is the A. raridentata of Searles
Wood, who has recognized, in his work on the Crag
Mollusca, the priority of Scacchr's publication, and
adopted the name which I have now given ; and it ap-
pears to be also the A. pusilla of Nyst.
B. Shell : equivalve teeth numerous and uniform, set across
the hinge -plate, and either divided into two rows or
arranged in a single
and continuous row.
2. A. obli'qua*, Philippi.
A. obh'qua, Phil. Faun. Moll. Sic. ii. p. 43, t. xv. f. 2.
Shell obliquely oval, with a rhomboidal outline, much narrower
at the anterior side, and spreading out on the other side,
* Oblique.