192 



the basal edge. Some are covered by a kind of hairy epidermis. 

 Blainville says, that the animal has the body thick, of a somewhat 

 variable form; abdomen provided with a pedunculated, compressed 

 foot longitudinally divided ; mantle with a simple range of cirri, 

 and a little elongated before ; the tentacula are very small and 

 very slender. 



[Arca staminea, pi. 36, fig. 2, and Aeca lienosa, pi. 36, fig. 1, being 

 fossil species, I have omitted the descriptions. — Ed.] 



SiPHONARiA. — Shell oval or subovate, patelliform ; apex nearer 

 one side and one end, curved in a direction opposite to an angle 

 on the basal edge ; within, an indentation dividing the muscular 

 impression and extending to the angle of the lateral edge. 



Ohs. Adanson was the first to detect the differences between a 

 species of this genus, the S. mourefus, Bl., and Patella which it 

 resembles in general form. He was succeeded by Blainville, but 

 it is to Sowerby that we are indebted for the genus and name 

 which are now adopted. Gray formed a genus for the Gadin of 

 Adanson, which, however, is referred by other authors to the present 

 group ; Adanson did not describe its animal ; but it is evident 

 from his figure that the two sides of the shell are not symmetrical. 



It resembles Patella, but the shell is distinguished by an un- 

 symmetrical angle on one side, (sometimes obsolete,) denoting the 

 position beneath of the termination of a syphon or respiratory organ 

 of the animal of which the trace remains. On the outer surface, 

 corresponding with this mark, is generally an elevation or rib, ex- 

 tending from the summit to the edge, but it is sometimes obsolete. 



Blainville gives the following characters of the animal : body 

 oval subdepressed ; head subdivided in two equal lobes ; tentacula 

 and eyes indistinct ; margin of the mantle crenulated and extend- 

 ing beyond a suborbicular foot as in the Pafellas, branchial cavity 

 transverse, open a little before the middle of the right side and 

 provided in that part with a fleshy lobe, of a square form, situated 

 in the sinus between the mantle and the foot ; retractor muscle of 

 the foot, divided into two parts, of which the posterior is much the 

 larger, arquated ; the other very small, before the branchial orifice. 



Several species are known, attaching themselves to rocks and 

 other fixed bodies. _ 



SiPHONARiA ALTERNATA. — Patella altemata, nob. Jour. 

 Acad. Nat. Sc. vol. 5, p. 215. 



