68 LECTURES ON MOLLUSCA. 



the spire very small. Just as we found conical forms among the 

 Nerites, so we have a conical Trochid. It is called Broderipia, and 

 looks just like a small, pearly Limpet. 



Family PROSERPINIDJE. 



A curious little family of land shells are believed by Dr. Gray to 

 have the same relations to Nerita and Trockus that Cydostoma and 

 Helicina have to the Periwinkles. They differ from the true Pulmonates 

 in having the mantle free from the nape, leaving the breathing cavity 

 open. They differ from Helicina, &c., in having glassy teeth in com- 

 plex pattern like Trochus, and in having no operculum; in which 

 respect they resemble Stomatia. The mantle is unadorned, as in 

 Nerita; and, like it, has the power of absorbing the inner whirls of the 

 shell. On the other hand, it is said to be unisexual, in which it re- 

 sembles the Pectinibranchs rather than the present order. The group 

 is West Indian, and contains two genera: Proserpina, in which the 

 whole shell is glossy, like Pupina; and Ceres, in which it is keeled, 

 and only the lower region is polished. In both there is a lump on the 

 pillar, as in Rotella; and there are spiral ridges inside the mouth. 



Family SCISSURELLID^I. (Slit-Top Shells.) 



Till lately it was believed that there was no living representative of 

 the vast tribe of palaeozoic and secondary Pleurotomarias ; except the 

 tiny little shells of Scissurella, which resemble a Vitrinella with a slit 

 in the mouth, or a spirally curled Emarginula. The tiny animal has 

 been examined, and found greatly to resemble Cydostrema, having 

 very highly developed pinnate feelers at the sides. In some species 

 the slit of the young shell is afterwards closed into a hole; in others, 

 the hole is seen in the earliest stage, and is moved on as in Eimula. 



But the true Pleurotomaria, which was believed to have passed away 

 before the Tertiary age, is now known to be living, a beautiful speci- 

 men having been dredged in deep water near the island of Marie 

 Galante, so like the Oolitic forms that it might, if fossilized, have 

 passed for one of their race. It is exactly like a pearly Calliostoma, 

 with a slit lip. More than four hundred fossil species are known, 

 some of them as large and solid as the Turbos, some as inflated and 

 thin as Scissurella. In form they vary from Elenclius to Euomphalus, 

 and are either keeled or rounded at the base. In Trocliotoma there is 

 a hole behind the lip, instead of a slit. In Polytremaria there is a 

 row of holes in a spiral necklace, as in Siliguaria. The shells of the 

 palaeozoic group Murchisonia are elevated like a Melania; while those 

 of Schizostoma are depressed like a Euomplialus, with a doubly waved 

 lip like Terebralia. Another palaeozoic form, Catantostoma, has the 

 last whirl twisted downwards. The closely allied shells of Scalites and 

 Rapliistoma are very thin and depressed, with the whirls keeled and 

 the outer lip pinched but not slit. 



?? Family MACLUREADJE. 



Of several other palaeozoic forms, even the family position is as yet 

 doubtful. One of the most singular is Madurea, a Euomphaloid shell 



