OF SEPTA BISEBIALIS IN COENWALL. 101 



2^ inches, and the breadth (I suppose without the membranous 

 wing) was y^ths of an inch ; but in the most perfect of my spe- 

 cimens, which however is defective at its blunt end, the length is 

 4 inches, and the breadth, including the membranous border, 

 If inch. The form is much more slender than in the common 

 species, and, as it approaches the mucro or spiu', may be described 

 as lancet-shaped. The spur projects much more considerably 

 than in the Sejyia officinalis, although in the latter I find a differ- 

 ence in different specimens. In the 8. hiserialis it advances -i-th 

 of an inch beyond the membranous border in a straightforward 

 direction, whereas in the >S'. officinalis it is depressed and bent 

 inward. The membranous border at this part is also tiu-ned 

 inward, so as to enclose a cavity, and conceal the narrower portion 

 of the shell. The structm-e of this spur in the 8. hiserialis is also 

 different ; for while that of the common species is for the most 

 part hooked, round and simple, in the 8. hiserialis it is slightly 

 lobed or keeled. I have not been able to assure myself of its 

 shape at the point, as in my most perfect specimen it was a little 

 injured. 



In addition to these well-marked distinctions, the rarer species 

 is smooth on its dorsal aspect at that part where the more 

 common is covered with a decided graining. The transverse striae 

 are also much more numerous, and of a finer structure. Their 

 direction also at the sides is not so bent towards the slender ex- 

 tremity. 



It is still a subject of interesting inquiry whether the animal 

 to which this shell or plate belongs is truly a native of our own 

 waters, since it does not appear that any observer has ascertained 

 its existence. It is to be observed, however, that although tlie 

 whole of these specimens were injured, and most of them much 

 broken, they were scarcely more so than were those of the com- 

 mon species, which is, beyond question, common on our coasts. 

 The two species were lying together on the beach, as they had 

 been left by the tide ; and although there were at no great 

 distance from them many specimens of the stalks of the American 

 maize, clothed with perfect leaves — and although without heads of 

 grain, still bearing clumps of the male flowers, — yet we camiot con- 

 clude from the presence of these foreign plants that the shells 

 can have come to iis from a considerable distance, since those of 

 the Sepia officinalis certainly have not done so, and the leaves and 

 stalks of the plants bear no marks of long immersion or of a distant 

 voyage. It is pi-obable that they A\'ere thrown overboard from 



