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 prevaiency 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 to it. Besides, in the loAver half, which remains in my 

 possession, the dental plates, considerably reduced in width, do 

 not retain their original direction^ being 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 appendage f : 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, I 

 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. 



t 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 sinffk slice across the umbone. An example of the first result, obtained 

 in a specimen of Spirifer fn'andicostatm, is given in fig. 27, and of the 

 latter, in Sp. cuspidatus. in fia-. 23. 



2* 



