CORBULA. 183 



Common as this moUusk is, it is so excessively shy when 

 taken from its home in the sea-bed, and placed in a vessel 

 of sea-water, that it rarely exhibits under such circum- 

 stances either its foot or its siphons. This is probably the 

 cause of the very contradictory statements which have been 

 oifered respecting the animal of this genus, for, when ex- 

 amined in a preserved state, only a very slight lesion of the 

 tissues is apt to lead us to erroneous conclusions with re- 

 spect to its systematic position. Mr. Clark seems to have 

 been especially happy in his opportunities of observing the 

 creature, and his account of it is so much more complete 

 than either published statements or our own notes furnish, 

 that we cannot do better than give it in his own words. 

 " Animal subtriangular, thick, of a yellowish-white colour, 

 having the mantle closed, except anteriorly, where there is 

 an orifice, whose circumference is finely notched, for the 

 passage of a moderately long, narrow, but rather thick 

 foot ; posteriorly it forms two very short, united orifices, 

 with eight or ten rays each. The upper, or anal one, has a 

 tubular retractile membrane, which the animal often pro- 

 trudes and retracts. We presume the sudden protrusion of 

 this organ is to give force to the propulsion of the fasces. 

 On each side of the animal there is a pair of very unequal 

 branchiae, which hang very obliquely from the dorsal line, 

 the upper one being rather narrow, the lower much larger 

 and triangular. They are of a brown colour, and finely 

 pectinated. There are also two rather long and slender 

 labial tentacles on each side. The tubes are yellow, the 

 branchial one having a circle of red around its orifices, and 

 the upper only a red mark or two.'"* 



Ordinary specimens of this shell do not exceed half an 

 inch in length, and about one-fourth less in breadth. It is 

 most abundant, often occurring in immense numbers and 



