LECTURES ON MOLLUSCA. 183 



they are widely dissimilar. In dentition, they are nearly related to 

 the Volutes, having only a central row of teeth. But these, instead 

 of having three lohes, or a spike, are very broad, with nine small 

 serrations. The proboscis is short, I think ; the siphon without auri- 

 cles ; and the foot is folded up longitudinally, as in the Mitres. They 

 further differ from most of the Volutes in their high polish, caused 

 by the sides of the mantle folding over the shell. Sometimes it deposits 

 a large callosity on each side of the mouth. 



In the typical Marginellas, the spire is distinct ; the siphonal notch 

 is not sharply cut out as in the Volutes ; and there are five distinct 

 plaits on the pillar. They inhabit clear sands, in somewhat shallow 

 water, and glide along with great rapidity. In Persicula, the spire 

 is concealed ; the pillar has numerous plaits ; and the outer lip has 

 an excretory notch, and is generally grooved within. In Volvarina 

 the shell is very thin, scarcely thickened at the lip, and with very 

 small plaits on the pillar. Several small species of this group are 

 common in the West Indies. A group of small shells, called Closia 

 by Dr. Gray, are extremely like Cyprceovula in shape. The outer lip 

 is toothed, and the inner has two large and two small plaits. 



In the next group of families, the teeth are arranged in rows of 

 seven each ; the central an inner lateral teeth being fixed, as in Fas- 

 ciolaria ; but the two outer teeth on each side being movable. The 

 inner teeth have numerous serrations on the edges. They are gene- 

 rally very small and transparent ; but the animal makes up for their 

 minuteness by having a strong prehensile collar at the end of the 

 trunk. In this are inserted a number of horny plates, armed with 

 numerous rows of conical teeth. 



Family CassidjE. {Helmet Shells.) 



The true Helmets are large, handsome shells, somewhat triangular 

 in form, with very short spire, narrow mouth, toothed on each side, 

 and the canal suddenly twisted backward. Like the Murices, they 

 leave a varix outside the shell at every period of growth ; which, in 

 this genus, occurs at every two-thirds of a revolution. The animal 

 has a large strong foot ; and the mantle deposits a very thick pillar- 

 lip, the edge of which projects so as partially to conceal the spire. As 

 the shell grows, the twisted canal is covered over by the advancing 

 pillar lip, leaving a cavity behind. The creatures are active and vora- 

 cious ; crawling, with their stout helmet behind their heads, (a fashion 

 which ladies have sometimes imitated,) and their nose-pipe bent back 

 over it, along the sandy flats where the unconscious bivalves quietly 

 wait to be eaten. The inner lip consists of various plates of enamel, 

 which lie in alternate colors. Artists have taken advantage of this to 

 carve cameos ; which are produced by cutting the figure in one of the 

 layers, and leaving the groundwork in the next. The large cameo- 

 shell, called by Lamarck Cassis madagascarensis , is a native of the 

 Bahama Islands, whence large quantities are brought to the Liver- 

 pool market. Dead shells have been dredged by Dr. Stimpson off the 

 coast of North Carolina. The colors of the cameos differ according to 



