6 1 2 Natural History of Molluscous Animals : — 



fully of its constitution and formation [Letter 5., in Vol. III. 

 p. SS5 — 348.], that at present I shall confine myself to some 

 additional remarks on internal shells, or such as are constantly 

 covered by the cutaneous envelopes. So far as I remember, 

 all of these are white, and generally thin ; but they do not 

 diifer from other shells in their composition, unless we include 

 among them the horny plates which strengthen the back of 

 some Cephalopodes (Loligo), or protect the viscera of a few 

 of the Gasteropodes (Limax) ; for these contain very little, if 

 any, cretaceous matter. The plate of the slugs, which lies 

 under the shield on the anterior part of the body, has been 

 called the snail's stone, and used to be esteemed a serviceable 

 medicine in the gravel and strangury. It varies in size and 

 consistency in the different species ; a fact of which Swammer- 

 dam does not seem to have been aware ; for, to account for 

 large slugs having " very small and membranous " plates,; 

 while smaller ones had them often much larger, and formed- 

 of " solid stone," he was inclined to think " that the snails 

 change this their little stone yearly, in the same manner as 

 crawfish change those two semiconvex and plain stones, which 

 are likewise placed in their thorax, and are improperly called 

 crab's eyes." {BooJc of Nature^ P« 71.) But the most interest- 

 ing of internal shells are, the Spirula PeronzV of Lamarck, 

 and what is commonly called cuttlefish bone. The former 

 (j^*-, 104. Z>) is a pretty multilocular species, rolled up in a 



spiral form, which lies partially 

 embedded in the posterior part 

 of a Cephalopode {a) that inhabits 

 many warm seas ; the latter is a 

 calcareous production, of so sin- 

 gular a structure, that, according 

 to Cuvier, it is unexampled in 

 the animal kingdom, and may, 

 therefore, merit a detailed de- 

 scription. The bone is placed 

 in the back of the cuttlefish (*Se- 

 pia officinalis), within a mem- 

 branous capsule, from the inner surface of which it is secreted, 

 but with which it has no connection further than a slight 

 adhesion from contact ; for neither blood-vessels nor nerves 

 penetrate the interior, and it is truly as unorganised, and 

 as little within the influence of the living powers, as other 

 shells are. In the very young »Sepiae, the bone is horny; but 

 in the mature cuttle it is entirely calcareous, if we except ar 

 thin membranous border. It is very light, of an elliptical 

 figure, upwards of 6 in. in length, and not quite 3 in. in 



