LECTURES ON MOLLUSCA. 209 



tened whirls, oblong mouths, and waved outer lip. They are like 

 strong marine Melanias, and are found in Greenland, Africa, and the 

 Eocene tertiaries. Eglisia has a deeply-marked suture, small mouth, 

 and thickened pillar. Shells apparently belonging to this family are 

 found in very old rocks. The typical forms begin in the neocomian 

 strata, and are exceedingly abundant in the tertiaries. Among the 

 latter is the genus Proto, in which there is a broad notch near the 

 front of the pillar. The shells of ScoUostoma, which range from the 

 Devonian to the Trias, form a remarkable transition to the Vermetids, 

 the aperture being produced and trumpet-shaped. 



It is difficult to say what are the true relations of the 



Family CLecidjEj 



whose tiny shells, like bent tusks, closed at one end, are seldom seen 

 in the cabinets of collectors, but present many points of singular inter- 

 est to the inquirer. The C cecum is first born as a flat spiral shell, 

 like SJcenea with which indeed the animal has not a few relations. 

 But after making two or three turns, it suddenly leaves the spire, and 

 grows outwards in a very slightly arched curve. In this state it 

 remains permanently in Strebloceras , the earliest Csecids known, from 

 the London Clay ; like a shepherd's crook, twisted at one end into a 

 spiral. But in the living genera, it soon drops off the spire, plugging 

 up the broken end ; and as it advances in growth, it brings the plug 

 forward, and drops off the part behind, always living in a part about 

 the same length, broader in proportion as it approaches maturity. In 

 Ccecum proper, the shell advances in the same plane ; so that if all the 

 decollated parts had been preserved, the whole would have had some- 

 what the shape of a Spirilla. In the West Indian genus Ifeioceras 

 however, where the shell has to keep pace with the growth of the 

 sponge among which it lives, the coil is in loose cork-screw, like a 

 drawn-out Turritella. The animal agrees with Turritella in having a 

 short foot and many-whirled operculum : also in partitioning off its 

 forsaken portions. But the division, instead of being a homogeneous 

 septum, continually repeated, -as in the Screws, is a very curiously- 

 shaped plug, the form of which is constant in each species. The teeth, 

 instead of being broad, with fine serrations, as in the Screws, are said 

 to be pointed and hooked, as in the carrion-feeders. As they areprin- 

 cipally found in worm-eaten passages of dead shells, they may be 

 employed as scavengers, to scrape up the decaying matter that might 

 otherwise corrupt the water. The adult shell has both its mouth and 

 plug slanting, so that it may be able to crawl through a very narrow 

 hole. In the earlier stages, the shells of all the Caeca are smooth and 

 slender ; but as they attain maturity, the group Anellum develops 

 concentric rings, the Elephantidum longitudinal furrows ; while the 

 shells of Fartulum are smooth, and look like tiny sausages. In Bro- 

 china, the plug is spherical, and the operculum swelling outwards. 

 The Cascids culminate in tropical America, east and west ; and are 

 curiously rare in the Pacific ocean. 



14 



