72 CLASS 1. TROPIOPODA. ORDER I. BIMUSCULOSA. 



generally oblong, inequilateral, rather inequivalve, and gaping at the 

 anterior side. In the young shell the hinge is sometimes furnished with 

 two indistinct, minute, obtuse teeth, but in the adult they become obso- 

 lete. The ligament is external, and there is no sinus observable in the 

 muscular impression. 



Example. 

 PL L. Fig. 1 to 4. 



Saxicava rugosa, Lamarck, Anim. sans vert., new edit., vol. vi. p. 152. 



Mytilus rugosus, Linnreus. 

 Byssomya (rugosa?), De Blainville. 

 Hiatella arctica (testa junior), Lamarck. 

 Solen minutus (testa minuta), Lamarck. 



P/wtas to the solvent powers of an acid secretion softening the rock, which would afterwards 

 be worn away by the rasping action of the shell; those produced by the land-snails he also 

 attributed to the action of acid secreted in very small quantities by the foot of the animal 

 during its daily retreat to these habitats, through very long periods of time. Apertures 

 such as these, occurring at great heights and far inland, could therefore no longer be adduced 

 in proof of elevation. 



" Prof. Owen objected to the mode of accounting for the perforations of the PItolas and 

 other boring Mollusca by acid secretions, because rocks which are not calcareous are also 

 bored by bivalves. He attributed the formation of these cavities to the constant current of 

 water around the shells, produced by extremely minute, vibratile cilia, which move inces- 

 santly, and independently of the will of the animal. The cilia cover the whole of the gills 

 (branchiae) and other parts of the animal, and produce currents in the water necessary to its 

 existence ; when lodged in the rock, the currents increase in intensity as the shell proceeds 

 inwards. 



" Mr. J. Phillips considered the small holes covering the surface of some of the rocks due to 

 a different cause from the deep excavations occupied by the Pholas and Saxicava ; besides 

 the boring Mollusca, several other animals are known, which have a characteristic mode of 

 perforating rocks. He alluded to the beautiful regularity of the holes made by the Pholas 

 in proof of their being formed by the shell, and not by currents of water. 



" Mr. De la Beche observed, that free carbonic acid, applied to the limestone, will convert it 

 into a bicarbonate, soluble in water, and the animal might apply the acid exhaled in breathing 

 to the purpose of softening the rock. 



" Dr. Buckland, in reply to Prof. Owen's remarks, observed, that if the perforations were 



