Prof. W. King on Spirifer cuspidatus. 19 
extending from the hinge to the opposite surface of the um- 
bonal cavity, are present, though concealed as it were by a 
shelly deposit occupying the interspace between them. My 
rubbing of this species also shows what I take to be indica- 
tions of the canal, this part being apparently represented by a 
dark spot (d) in the centre of the arch. The same appearance 
has occurred to me in the two specimens I have examined. 
The various examples, representing a number of species, 
which have now been brought forward have an important 
bearing on the question raised by Dr. Carpenter’s conclusion, 
inasmuch as the prevalency of the canaliferous septum plainly 
shows that this part is an essential structural element ; this cir- 
cumstance, moreover, strongly invalidates the idea that such a 
part characterized one species of shells, and was totally absent 
in another, the two being “ not distinguishable from each other 
by external conformation.” Apparently it has not occurred 
to Dr. Carpenter that the absence of the canaliferous septum, 
in Mr. Davidson’s specimen, may be due to some accidental 
cause. 
Mr. Davidson having kindly permitted me to operate on the 
specimen, I carefully ground down the upper half of the umbo- 
nal portion, sliced off by Dr. Carpenter, until scarcely anything 
remained. No canaliferous septum was exhibited, but there 
occurred to me some isolated platy fragments, which I suspect 
belonged toit. Besides, in the lower half, which remains in my 
possession, the dental plates, considerably reduced in width, do 
not retain their original direction, beimg much more inclined 
towards each other and to the plane of the area than is usual 
(fig. 30*)—a circumstance strongly in favour of their having 
been disturbed by pressure. Considering all points, I cannot 
but believe that Mr. Davidson’s specimen, which disclosed to 
Dr. Carpenter “ no vestige whatever” of the canaliferous sep- 
tum, has lost this appendaget: it appears to me to have got 
detached from the dental plates, either before the inside of the 
shell became completely filled with mud, or before the mud 
got hardened. 
With respect to the internal structure of the small valve, | 
have nothing to add to the account already given of it by 
* This must be accepted as a perfectly exact representation of the 
dental plates, as the figure is a facsimile of an impression obtained from 
them and the outline of the valve. 
+ In consequence of the somewhat variable length of the canaliferous 
septum relatively to the dental plates in their middle, some specimens 
might show nothing, or no more than a faint trace, of the canal, on making 
a single slice across the umbone. An example of the first result, obtained 
in a specimen of Spirifer grandicostatus, is given in fig. 27, and of the 
latter, in Sp. cuspidatus, in fig. 25. 
D% 
a 
