18 Prof. W. King on Spirifer cuspidatus. 
At first I could only be certain about the existence of the 
septum in its simple form. It was divided in the middle, or, 
in other terms, composed of two portions, each projecting from 
the inside of one of the dental plates, and separated from each 
other by a considerable interspace (fig. 24). Finding it ina 
similar condition in another specimen, I began to suspect that 
the component parts always remained disunited, as prevails in 
Spirifer mosquensis. In one specimen the division presented 
some appearance of being an incomplete canal ; but, not being 
certain, [ rubbed down the umbone of another specimen, two 
inches in width, and thus exposed the septum, well developed 
and enclosing the canal, as in fig. 25. I could have no doubt 
whatever that in this case the septum was canaliculated ; but, 
in order to place the matter beyond dispute, I gave the speci- 
men a further rubbing, which, though resulting in the oblite- 
ration of the circular form of the canal, confirmed the view I 
had taken, by disclosing it as a narrow slit dividing the sep- 
tum (fig. 26). I have obtained corresponding results by simi- 
larly operating on some other species*. Spirifer Verneuili, 
from Boulogne, has the septum; but the canal is feebly indi- 
cated, just as it appeared in the second rubbing of Professor 
Harkness’s specimen of Sp. cuspidatus. In Sp. grandicostatus, 
M‘Coy, both are tolerably well displayed, the canal being 
situated on the back of the septum ; the next rubbing entirely 
removed the canaliferous septum, leaving nothing but the 
dental plates extending right across the umbonal cavity (fig. 
28). This was also the case in Sp. striatus and some other 
species: sometimes the outer half of the dental plates disap- 
peared along with the septum. Sp. crassus, De Koninck, 
showed the apophysis somewhat obscurely at first; but on 
further rubbing, it became quite distinct (fig. 29). 
In all the species last mentioned the space between the 
dental plates above the septum is simply filled with crystalline 
or amorphous mineral matter. In some others the same space 
contains a shelly deposit, which is particularly the case in the 
Permian Sp. alatust. At first the latter circumstance seems 
to favour the idea that the arch is simply an excavation in the 
shell-substance of the pertaining valve. But this cannot be 
the case; for, as will be seen in fig. 9, the dental plates (3, 5), 
* The canaliferous septum is confined to the incurved or apical portion 
of the umbone, on which account it is with difficulty detected, and only 
by a close and frequent examination of the different surfaces obtained by 
rubbing. The canal is not often distinctly seen. 
t The arch in this species does not always contain a shelly deposit, as 
casts before me, from Humbleton "Hill, show that the inside of it has 
been quite divested of any extraneous infilling. 
