390 MOLLUSCS. 



Choanopoma, Cistula, Tudora, Omphalotropis, are the principal forms of Cyclo- 

 stomatidai. Some of the handsomest and largest species are found in Madagascar 

 and Mauritius, but some of the West Indian islands are noted for the immense 

 number of species they produce. Among the Cyclophoridce, the most important 

 groups are Cyclophorus, Leptopoma, Cyathopoma, Pterocyclus, Opisthoporus 

 Pupina, Cataulus, and Megalomastoma. The minute forms, known as Opistho- 

 stoma are, perhaps, among the most wonderful structures of the whole family. 



The species comprised in the family Truncatellidce are all small, and live either 

 between tide-marks (Truncatella) or upon the land (Geomelania). The shells are 

 elongate, and in the young state terminate in a pointed spire, which is subse- 

 quently cast off", the shells then assuming a truncate appearance at the top. They 

 have a very peculiar mode of progression, resembling that of the looper cater- 

 pillars, the end of the muzzle and the foot being successively advanced. The 

 characteristic feature of the typical genus (Hipponyx) of the allied family 

 Hipponychidce is the secretion by the foot of the animal of a shelly plate, which 

 is attached by its outer surface to stones, shells, or other substances. It forms a 

 permanent resting-place for the conical shell, and is attached to the adductor 

 muscle of the animal. In some of the fossil species these lower plates are so greatly 

 developed that they were at one time regarded as bivalved shells. Amalthea has 

 a shell very like Hipponyx ; it does not form a basal plate, but excavates a hole 

 in the surface of other shells to fit the aperture of its own. 



The members of the family Calyptrceidce are limpet-like in their mode of 

 life, living attached to stones, shells, etc. The shells are more or less conical, but 

 spiral towards the apex. The interior is either simple, as in Capulus, parted off 

 by a transverse septum (Crepidula), or a cup-like process, varying in form, is 

 developed in the upper part (Crucibulum). The animals have a short proboscis, 

 two slender tentacles with the eyes near the base, and the mouth furnished with 

 a radula bearing a prolonged tusk, like the Naticidce. Being permanently located, 

 it becomes a matter of conjecture what they feed upon. Possibly the minute 

 forms of life which abound in the sea form their staple nourishment. The various 

 genera are for the most part recognisable by differences in the shell, the form 

 of the internal septum and " cup " furnishing good characters. The number of 

 living forms is considerable, and they are found in all seas. Two occur on the 

 British coast, — Calyptraia chinensis and Capulus hungaricus, — and a third, 

 Crepidula fornicata, supposed to have been imported with American oysters, is 

 becoming an established resident on the Essex coast. Closely allied to Capulus 

 is Thyca crystallina, which lives parasitic upon star-fish at Mauritius and in 

 other parts of the Indian Ocean. In the family Xenophoridm are contained a 

 remarkable group of molluscs known as carrier-shells, so-called from the instinctive 

 habit some of them possess of carrying about with them shells, stones, and other 

 substances, which they cement to the exterior of their own trochiform shells. 

 Doubtless this concealment is to some extent protective. The animals bear some 

 resemblance to Strombus, but their eyes are very inferior, and placed at the base 

 of the tentacles. They are furnished with opercula, which, however, are more 

 like those of Purpura than Strombus. They resemble the latter in their leaping 

 and scrambling mode of progression, not gliding along on the sole of the foot 



