CHAP. VII. TROCHIDvE. ACHATIN.E. 227 



These analogies, of course, are much more remote 

 than those we have heen tracing, because the groups 

 themselves are much more remote; and yet the same mode 

 of variation is preserved in two different families one 

 marine, the other terrestrial. The Rotellce in one, and 

 the Cyclostomce in the other, are the most discoid shells 

 in their respective families ; while, at the opposite side 

 of the circle, we find the long-spired Phasianelke repre- 

 senting the still longer-spired Clausilice. Some of the 

 Helicina have a deep narrow slit at the hase of their aper- 

 ture, precisely similar to that on the outer lip in Pleu- 

 rotomaria ; and in both instances this structure is only a 

 repetition of what is seen in lanthina, Scissurella, and 

 Pleurotoma. The two first analogies equally hold good, 

 so that the two groups mutually test each other. 



(212.) We have placed the Trochida next to the 

 Helicida, under the belief that they followed each other, 

 although the links of connection were wanting. It is 

 clear, that of all the types of the TROCHID.E, Rotella is 

 that which by its general form makes the nearest ap- 

 proach to Helix ; while the thickening of the inner lip, 

 which spreads over the umbilicus, is found also, but in 

 a less degree, in many of the land volutes, Lucernincc. 

 But a singular discovery, recently made, has thrown an 

 entirely new light upon this interesting question. Among 

 a considerable number of freshwater Planorbi, all of one 

 species, which were sent us from Brazil, we picked out 

 two helix-looking shells, so precisely of the same olive 

 brown colour, and of the same size, as the others, that 

 none but a conchologist would have been led to examine 

 them. They appeared, in fact, like two little land- snails 

 of the sub-genus Zonites, that had fallen into the water 

 where the Planorbi had been found, their outside being 

 discoloured, and covered with little particles of dirt and 

 sand. On placing them, however, under the magnifier, 

 a conchologist can alone judge of our astonishment at 

 finding that the whole of the shell was actually com- 

 posed of little stones and grains of sand only, agglutinated 

 together, yet with so much skill, by the animal, that the 

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