91 



fold : brachial fold developed very rapidly from the base, in 

 broad and deeply undulating convolutions, so deep as to leave 

 no regular central cavity; most prominent surfaces much 

 flattened, and spread laterally till those of adjoining convolu- 

 tions meet and unite in many places, often presenting, towards 

 the lower part of the whole, a continuous surface. 

 This species displays many peculiarities. The structure of its 

 simple membrane seems coarse, that is, the squares are larger 

 than in any other species of the Ventriculidse. The root is ex- 

 ceedingly short. The most curious point, however, is the union 

 of the flattened prominences of the convolutions. A reference 

 to fig. M, p. 89, will show, in B. tuber osus, a disposition to 

 flattening of the most prominent part of the convolutions in 

 that species, and will make the nature of the present fold easily 

 understood, where that flattening is so much more extensive 

 that adjoining convolutions meet and unite*. This peculiarity 

 was important in the present species on account of the great 

 depth of the convolutions, which would, without it, have been 

 more liable than in B. eleyans and other species to be displaced, 

 and so injury to have happened to the polyps. But a curious 

 phenomenon is often presented in consequence. The flint, 

 which, from its specific gravity, would always lie, when fluid, 

 near the sea-bottom, was attracted round the base of the spe- 

 cimen ; but, the surface there being usually continuous for a con- 

 siderable distance, access to the lower part of the inside seldom 

 took place, unless the siliceous fluid was very abundant ; any silex 

 attracted towards the inside f in the manner before suggested 

 (p. 13) being specially attracted by the convolutions interposing 

 before it reached the base, solidifying there, and thus preventing 

 the flow of other fluid towards the base. Hence, on the decay 

 of the animal matter, a large hollow was frequently left in the 

 flint, which is often now found only partially filled up by chal- 

 cedony. Such flints are cup- shaped, very regularly rounded be- 

 low, and have a flat top. Near the edge of that top a continuous 

 line is usually found, showing the place of the united mem- 

 brane. Within this are seen traces of the convolutions of the 



* This outer surface, where the polyp-skin is preserved, which very rarely 

 happens, may sometimes appear as if apolypous. A very careful comparison 

 of all the specimens I have, on which remains of this membrane are found, 

 leads me, however, to the conclusion that this is not the fact ; but that, the 

 membrane being coarser than usual, the polyp-cells, though present, are not 

 so clearly preserved as in some other species. 



f The access of the liquid chalk was often made equally difficult by the 

 collapsion, after death, of some of the lower convolutions on one another. 

 Hence we find the bases of specimens enveloped in that substance often hol- 

 low like those in flint. Some deposit of chalcedony has often taken place 

 in those hollows, as in the hollows of the flints. 



