CHAP. VII. SOLARIUM. ONUSTUS. 221 



shape more depressed and trochiform, and the surface of 

 the shell often granulated ; the only remnant of the tooth 

 is shown by one or two small tubercles or notches at the 

 base of the outer lip. Several species^ mostly of a small 

 size, are now before us, all of which are natives of warm 

 climates or of the Pacific Ocean. Lastly, we find in 

 this genus, as well as in Trochus, one division which 

 have not perlaceous shells, but which, possesses a 

 well-defined tooth. These species we include under the 

 name of Echinella. The most typical is that figured 

 in the Ency. Meth. (pi. 417- fig. 6.) as Monodonta coro- 

 naria, — a shell which is a perfect prototype of our Pa- 

 godella echinata : the species yet known are few; but of 

 these we possess the connecting link to Elenclms, in our 

 E. gramdata. We are thus brought back to Elenchus, 

 where we commenced our survey of Monodonta. The 

 naturalist will not fail to perceive, that in thus resting 

 our arrangement upon affinity, we have indicated strong 

 relations of analogy between Trochus and Mono- 

 donta : to these we shall presently return. 



(206.) The two remaining genera. Solarium and 

 Onustus, do not comprise more than a few recent species, 

 although it is highly probable that many of the imper- 

 fectly preserved fossil discoid shells belong to the first, 

 and some few also appertain to the last. On receding 

 from the typical species of Solarium, the spire becomes 

 more prominent, and the edge of the body- whorl dilated 

 to form a sharp edge, so thin as easily to be broken off ; 

 the crenated margin of the umbilicus of these shells, how- 

 ever, indicates their affinity to the singular genus Onustus. 

 From certain slight and irregular indentations on the 

 preliminary whorls of the spire, we suspect that, at an 

 early age, many of these animals gather small fragments, 

 and fasten them upon their shells ; but that, when older, 

 these extraneous substances either fall off, or are volun- 

 tarily discharged; just as if the animal, having acquired 

 its full powers by age, was able to fabricate its own ha- 

 bitation without calling in other assistance. Now, the 

 same advance which we have thus stated as taking place 



