TESTACEA. 71 



I have not soon any of the bivalve* endowed with a similar faculty 

 in shifting them by the same expedient. Those with a foot uae it in 

 crawling, if it is of sufficient consistence, power, and dimensions. In 

 many, however, theae conditions seem to be deficient. 



PLATE XVII. 



Fio. 8. TMna. 



6. VENUS TIBOUTEA (Tapes puUastra}.P\&le XIV., Figs. 4, 5, 6, 7. 



The shell, of irregular outline, may be circumscribed by a broad oval 

 of an inch and a half by an inch. It is of light brown colour. The 

 animal, nearly white, protrudes the anterior nearly two inches, the ex- 

 tremity forking into two cylinders, with a fringed orifice in each. A 

 broad thin foot, seldom seen, protrudes behind. 



This subject is affixed by a strong light coloured silky byxtus to solid 

 substances. 



It seems to be from inconvenience that the animal retreat* and 

 closes itself up in the shell. Its greatest expansion is at night. But it 

 is induced to protrude during the day by covering its vessel. 



PLATS XIV. 



FIGS. 4, 5, 6, 7. FMMU Viryiiua (Tape* puUwtra). 



VENUS CASINA. Plate XIV., Fig. 9. 



This shell is by no means uncommon along the Fife coast, and about 

 the Isle of May ; but it is rarely found alive, at least I have found it so. 



A living specimen was received on the 22d April 1845, and con- 

 tinued to live until the 17th April 1848. 



It had ceased to open the shell for months. Nevertheless, I did 

 not think the animal dead, from the weight of the shell. But it shewed 



