8 Mr. W. Clark on the Terebrating Mollusca, and on the 



doubted, and they are deposited partially as in its congener, in 

 the branchial tube ; the palpi and liver exhibit no variation. I 

 now come to the most decided difference between the two ani- 

 mals ; the foot, in the form we are now describing, is propor- 

 tionally larger than in any other of the Pholades, of hyaline tex- 

 ture, springing from the centre of the body with a long cylin- 

 drical pedicle ; it has a subclavate appearance, truncate at the 

 terminus, which is of suboval form and pointed anteally and pos- 

 teally, and there is no outward visible trace of the curious elastic 

 stylet common to all bivalves, and so conspicuous in the ventral 

 tissue of the form Pholadidea papyracea. 



I will now make a short comparison of the two forms : it will be 

 observed that it is stated, in the form Pholadidea papyracea, that 

 the mantle is closed, except a very small aperture or " spiracle " 

 for the foot, if it still exists ; but in the form Pholas lamellata 

 there is a large aperture for a foot, that is, larger in proportion 

 than in any of the Pholades. The branchiae, palpi, and elabo- 

 rate siphonal apparatus are precisely the same with only varia- 

 tions of colour ; the bodies of the two are of the same shape, but 

 differ in colour and markings, the one being intensely mottled, 

 the other hyaline ; the body of the one having no foot attached 

 to it, but the other a very large one. These are the principal 

 variations, and certainly constitute a very general difference of 

 aspect between the animals of the two forms, and it must be ad- 

 mitted that conchologists and even malacologists, who have not 

 examined with care all the conditions and incidents attached to 

 them, have had a prima facie case for doubting their identity ; 

 but notwithstanding these great and visible discrepancies, I think 

 I shall, by a suite of facts, observations, and reasoning thereon, 

 be able decisively to settle their specific identity. 



But before I apply to this discussion, I propose to communi- 

 cate what I consider to be the real agent of the Acephalous Mol- 

 lusca in the operation of excavating their dwellings. This abrupt 

 inroad on a subject only just mooted, will however, from the facts 

 adduced, shorten the discussion when we revert to the subject 

 we have for the moment abandoned, as they will I think satis- 

 factorily account for some of the great variations of aspect be- 

 tween the Pholadidea papyracea and the so-called Pholas lamellata 

 and other apparent anomalies. I disclaim all merit for the great 

 discovery of the animal functions that are the principal agents 

 of the excavating powers of the Acephala, and which will I think 

 for ever set at rest the endless discussions thereon, by placing 

 the subject on the indestructible bases of certainty. 



This great result is due to the genius and talents evinced by 

 Mr. Albany Hancock, in his paper in the i Annals ' of October 

 1848, " On the Boring of the Mollusca into Rocks." If any con- 

 siderations are due to me, they are of the most negative character, 



