and on the removal ofpoi'tions of their Shells. 237 



is most conclusive, for a better idea could not be given of the 

 peculiar concavity of the bottom of the burrow than by an allu- 

 sipn to this object. 



The foot of P hoi as crispata is like that of Teredo : it projects 

 in front of the valves, is very large and convex in front, agreeing 

 also in shape with the bottom of the burrow, to which it closely 

 and constantly adheres. The free borders of the mantle surround 

 this organ, and together with it almost fill up the bottom of the 

 excavation, the concavity of which so exactly corresponds to the 

 convexity of those parts that the one might be supposed to be 

 moulded on the other. P. Candida and P. parva have the foot 

 and anterior portion of the animal formed in the like manner, 

 being only specifically modified; and, judging from analogy, 

 there can be no doubt that all the individuals of this group are 

 constructed after the same type. P. papyracea and P. striata, 

 from being closed in front when mature, might be thought to 

 have the animal differently formed. But we have the authority 

 of Mr. G. B. Sowerby for asserting that P. laminata is the young 

 of the one, and P. minuta of the other. If this be the case (and 

 having carefully examined specimens of them all, I am inclined 

 to the same opinion), the animal of these two species when young 

 will undoubtedly resemble that of P. crispata, and the excavations 

 must be made whilst in that state. A¥e have already seen that 

 the burrows of all the boring bivalves are formed during the 

 growth of the animal. And in these two species after the bur- 

 rows are completed the anterior gape of the shell is filled up, and 

 probably the large foot greatly reduced. 



Thus then we see that in Teredo and Pholas the anterior por- 

 tion of the animal corresponds in form to that of the bottom of 

 the excavation. How is this in Saxicava rugosa ? Precisely the 

 same. The parts however are modified. 



The animal of this species, and of that of Gastrochcena Pho- 

 ladia, are alike : they are both closely enveloped in the mantle, 

 having only siphonal orifices, and a small perforation about one- 

 third of the length of the shell from the anterior end for the 

 passage of the byssus and a small slender foot, which is occa- 

 sionally protruded. The mantle in front is much thickened, and 

 forms a sort of cushion-like swelling of an elliptical form, extend- 

 ing some way backwards, and which at the will of the animal can 

 be thrust out considerably in advance of the valves. The arch 

 of the front of this cushion corresponds to the concavity of the 

 bottom of the burrow. As before stated, this part of the animal 

 has not the power of adhering to the substance to be eroded, 

 but is held in contact with it by the attachment of the byssus ; 

 and, howsoever the excavation may be effected, there can be no 

 doubt that this thickened portion of the mantle is the instrument. 



