44 LECTURES ON MOLLUSCA. 



probably also in Rollus geograpliicus, which differs from the rest in 

 having no operculum. The Cones are found in all tropical seas ; but 

 abound most in the Indian Ocean and Eastern Archipelago. Some of 

 the species are very widely distributed, reaching from the Eed Sea to 

 Easter Island and the Gallapagos. They prowl in the holes and fissures 

 of rocks, and the winding passages of coral reefs ; where they crawl 

 slowly in depths ranging from low-water mark to forty fathoms. The 

 shells are generally heavy : and as the animal grows stouter, he absorbs 

 the inner whirls of the shell, leaving only a very thin partition. At 

 the same time he preserves his weight by depositing thick coats in the 

 region of the spire. Shells therefore which a " collector' 7 would throw 

 away, may be valuable to grind down and show the inner structure. 



The Dibaplms is a puzzling shell, intermediate between Conus, Mitra, 

 and Terebellum. Its true position cannot be stated without a knowledge 

 of the animal. Fossil Cones first appear in the chalk : and are toler- 

 ably common in the tertiary strata. The Conorbis of the London Clay 

 is lozenge-shaped, closely approaching in form some members of the 

 next family. 



Family PLEUROTOMID^:. 



In this family the head is truncate, without a funnel. The shells 

 are generally turrited, and are only known from Fusus by a slit in the 

 outer lip, near the suture, corresponding with a slit in the mantle of 

 the animal. The typical genus, Pleurotoma, has a long canal, and the 

 slit separated from the suture. The operculum is flat, somewhat trian- 

 gular, with the nucleus near the canal. Drillia differs in having a 

 short canal. These forms are peculiar to tropical regions. They are 

 represented in Northern seas by Bela, which has a somewhat similar 

 operculum ; but the slit is nearly obsolete, and the pillar is flattened. 

 Lacliesis has a Mamilated spire, and a Buccinoid shape. 



Another group is characterized by the nucleus of the operculum being 

 in the centre of the long side, as in Pusionella and Bezoardica. In 

 Clavatula, the canal is short ; the shell resembling Drillia. In To- 

 mella, the spire is short and the canal produced ; the shell resembling 

 a Clavella, with a wave near the middle of the outer lip. There is a 

 thick deposit near the suture, as in that genus. 



A third group has no operculum at all. The ClatJmrellce, (Defrancia 

 of Millet ; the true Defrancia being a Polyzoon,) are among the most 

 beautiful of small shells. They are like a Drillia, with a deep posterior 

 notch close to the suture ; and the whirls are swollen and delicately 

 cancellated. They are found in temperate as well as in tropical cli- 

 mates. In Mangelia, the notch is very slight, and the shell plain ; 

 being in fact a Bela without an operculum. The Citharce are a group 

 of beautiful little shells, like flattened Harps. They have regular trans- 

 verse ribs, notched at the suture : the mouth is narrow and straight, 

 toothed or wrinkled within, like Oniscia. Dr. Gray places them with 

 Cassis, their true position being of course uncertain till the animal has 

 been examined. In Daplmella the shell is thin and ventricose, very 

 finely sculptured, and with the family notch almost obsolete. The 

 shell is closely related to Metula t which has probably a Muricoid 

 animal. 



