LECTURES ON MOLLUSCA. 49 



a small scar. It serves therefore as a shield when the animal is in 

 motion, as well as a door when it is at rest. 



The eyes in the Strombs are remarkably well formed, being (like 

 those of the Cephalopods) more highly organized than in many fishes. 

 They have a distinct crystalline lens, with an iris differently colored 

 in different species. 



The shell also is very peculiar. When young, it resembles a cone, 

 with the spire more or less elevated, and a very thin lip. But as it 

 approaches maturity, it spreads out a great wing, which is gradually 

 thickened with layer upon layer from the mantle, till the shell is very 

 strong and heavy, and able to tumble over without injury, as the ani- 

 mal scrambles on the rocky shore. The pillar has a twisted canal for 

 the breathing pipe ; and near it is a very deep notch in the outer lip, 

 where the animal can save his head from a blow as the shell falls over. 

 The wing is further notched at the suture. 



The Strombus gigas, or "Fountain-shell" of the West Indies, fills 

 up the earlier whirls with solid matter, and sometimes weighs five 

 pounds. It is a favorite ornament in consequence of the delicate pink 

 color of the mouth ; and is used for cameo-cutting like the Helmets. 

 It is alas ! ground to powder wholesale, for the manufacture of the 

 finer kinds of porcelain; three hundred thousand having been imported 

 into Liverpool in one year, from the Bahama islands. 



The Scorpion-shells (Pteroceras) are like the Strombs when young : 

 but when mature, they develop six or more long claws, variously 

 twisted. In Rostellaria, the head-notch is close to the breathing canal, 

 and the spire is long. An excurrent canal generally ascends the spire, 

 and is sometimes long enough to twist over at the apex and come 

 down on the other side. In the aberrant group Terebellum, of which 

 only one species is now living, the shell is glossy, sharply truncated 

 at the base, without canal or notch, and with a sharp outer lip. The 

 operculum is very singular, having the appearance of a bird's foot 

 with claws. The creature, when taken from the water, will leap sev- 

 eral inches. In one of the Eocene species, the spire is rolled in and 

 hidden ; in another, a canal ascends the spire as in the Spindle- 

 Strombs. 



The fossil forms belonging either to this group or to Aporrhats 

 appear first in the Oolites. Nature might seem to have amused her- 

 self in the strange and varied shapes which many of them assume, 

 especially in the Spindle and Scorpion tribes. The true Strombs^ how- 

 ever barely appear in the tertiary age ; at present they culminate, 

 while the other forms are dying out. 



Family PHORID^I. (Carrier Top Shells.) 



Very different in the form of shell, but agreeing in the peculiar 

 shape " of the animal, are the Carrier Shells. They live on banks of 

 stones and dead shells, chiefly in the East Indies, over which they 

 scramble, stretching out their foot-pole, with the opercular arm^and 

 the long muzzle, like the Strombs. Their eyes however are very infe- 

 rior, and are placed at the bottom of slender tentacles. They have no 

 breathing tube, the shell being top-shaped. Contrary to the habit of 



