3360 THE MOLLUSKS OR SHELLFISH 



tropis, are the principal forms of Cydostomatidcz. 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 Cydophoridce , the most important groups are Cyclop/torus, Leptopoma, Cyatho- 

 poma, Pterocydus, Opisthoporus , Piipina, Cataulus, and Megalomastoma. The 

 minute forms, known as Opisthostoma 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 subsequently 

 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 caterpillar, 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 con- 

 ical 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 Calyptrcndce are limpet-like in their mode of life, 

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

 toward 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 bear- 

 ing 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 recognizable by differences in the shell, the form of the internal septum 

 and ' ' cup ' ' furnishing good characteristics. The number of living forms is con- 

 siderable, and they are found in all seas. Two occur on the British coast, 

 Calyptr&a chinenszs and Capulus hungaricus, and a third Crepidula fornicata, sup- 

 posed to have been exported with American oysters, is becoming an established 

 resident on the Essex coast. Closely allied to Capulus is Thyca crystallina, which 

 lives parasitic upon starfish at Mauritius and in other parts of the Indian Ocean. 

 In the family Xenophoridce are contained a remarkable group of mollusks 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 S trombus, 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 



