378 Prof. J. F. Blake on 



formed previously to the septa tliey pass. They are sprinkled 

 more or less closely by minute dark spots, with irregular 

 radiations, like the lacunse of a bone, which may have a forma- 

 tive function. Again, some of these laminee start from the inside 

 of the body-chamber remote from the last septum, as indeed 

 it is obvious they must do, as the nacreous surface formed by 

 them is not confined to the camerated portion of the shell. 

 We may conclude from this that they are formed by the 

 suj-Jhce of the front part of the body-covering or mantle, and 

 would be formed whether there were any septa or not. They 

 are also formed successively, and as they are not devoid of 

 animal substance, we may perhaps say they are at first 

 " chitinous " membranes subsequently calcified, and that they 

 do not grow by intussusception. The third layer is a thin 

 amorphous substance covering the whole of the interior of the 

 shell. It is excessively thin, and though quite invisible, if 

 present, in the earliest chambers, is well marked in the later 

 portion of the shell, especially where the septa abut on the 

 shell-wall. It is here seen between the septum and the shell, 

 completely separating the two structures. It is thus seen 

 that the statement by Mr. Bather ('Annals,' p. 306) — that 

 " secretion and exfoliation, beginning in the anterior region 

 of the shell-wall, proceed backwards to the suture ; thence, 

 centripetally over the septum, to the posterior margin of the 

 septal neck ; a membrane of the septum is therefore one with 

 a membrane of the shell-wall, and each complete membrane 

 is typically shaped like a funnel," — either is entirely 

 imaginary, or my observation, which I have just verified 

 again, is erroneous. 



The structure of the septum is difterent from that of the 

 shell, though it is on the same type as the nacreous layer. It 

 is composed of a large number of equally fine laminse, also 

 speckled with lacunas ; but the laminee do not crop out on the 

 surface, but are parallel to it ; there is therefore no diffraction 

 of light, but a peculiar pearly lustre, due to the minute 

 floating specks, just as in a pearl, or at least as in an artificial 

 pearl whose lustre is similar to that of a natural one. It thus 

 difi"ers from the nacreous layer, just as pearls do from mother- 

 of-pearl. I judge this to have been formed in the same way, 

 by secretion from the surface of the body-covering in contact 

 with it. 



These observations do not lend much countenance to the 

 idea that the laminae in the pad of the Sepia are homologous 

 with the septa of a Nautilus, nor, therefore, with those of the 

 phragmocone of the Belemnite. If one might venture a 

 guesSj in the absence of more precise proof, it would appear 



