42 LECTURES ON MOLLUSCA. 



outside, and the lip periodically thickened within. The plaits are 

 very strong, often projecting heyond the mouth. {Sometimes there is 

 only one stout fold. In Odostomia, the sinistral apex is very small, 

 the shell Kissoa-shaped, with one tooth on the columella, which some- 

 times (as in Auricidina) becomes obsolete. In Monoptygma, the fold 

 is slanting.* In Chrysallida, the shell is strongly sculptured, and the 

 shell contracted at each end. The outer lip in the adult is extremely 

 thin in front, but thickened behind. The species are very numerous 

 on the west coast of America, where they are found in the crevices of 

 dead shells. All these creatures are very minute. The Chemnitzias 

 are somewhat larger, a few species actually reaching an inch in length. 

 They are very much turretted shells, with large sinistral apex and 

 melanoid mouth, without plait. Most of the species have flattened 

 whirls with transverse plaits ; but in Eulimdla and Menestho, they are 

 smooth. In Aclis, the whirls are tumid : and the mouth is sometimes 

 round, like a Scalaria without rings. 



Large shells are found in the palaeozoic and oolitic rocks, which are 

 referred provisionally to this family ; but the characteristic apex can 

 seldom be examined, and their true position is doubtful. In the ter- 

 tiary strata, we find representatives of most of the living forms. 



Very few species in this family abound in individuals, and from their 

 minuteness and rarity they are seldom seen in collections : but very few 

 families boast of so many specific forms. They are more numerous 

 even than Rissoas, both in the British seas and in the Grulf of Califor- 

 nia. Shell sand, especially from deep water, should always be care- 

 fully searched for them ; and the sinistral apex carefully examined, to 

 distinguish them from Rissoids, &c. 



Family SOLARIAD^I. (Perspective Shells.) 



The shells grouped together in the Trochus family by Lamarck, are 

 found to belong to five very widely separated groups. The true Troclius 

 is a Scutibranch, allied to the Ear-shells and Limpets. The Trochita 

 is a Kostriferous Pectinibranch, allied to the Slipper-limpets. The 

 Risella belongs to the Periwinkles, in the typical portion of the same 

 group. The Pliorus, or Carrier-trochus, belongs to the further extreme 

 of the same group, being a scrambler, allied to the Strornbs.. While 

 the Perspective Top-shells are found to possess a retractile proboscis, 

 and to have many -points of resemblance with the very differently shaped 

 Pyramidellids. The shell of Solarium is known by the wide open 

 umbilicus, which has always a crenulated keel within, ending in a 

 notch at the base of the mouth. The shell is top-shaped, with a flat 

 base, and is always beautifully sculptured. The point of the spire is 

 rather flattened, and there may always be noticed a minute hole, even 

 in perfect specimens. This is caused by the nucleus of the shell, which 

 is reversed and globular as in the Pyramidellids, being turned upside 



*This genus was constituted from very different shells. The supposed original type is 

 an abnormal Jlncillaria. The name is here kept, as by Woodward, for shining, sculptured 

 East Indian shells, intermediate in form between Odostomia and Tornatella. As the animal 

 has not yet been observed, their true position is uncertain. 



