MOLLUSCS. 



329 



Pig. 410. — OU- 

 vella biplicafa. 



is in motion, are folded up over the shell. The proboscis is short, the siphon long, and 

 the eyes are placed at about the middle of the tentacles. The eighty and odd known 

 species all come from the tropics, a few only extending their range outside. They are 

 all active, predaceous forms, and in some localities are caught by lower- 

 ing a net with a piece of meat inside as a bait. In some places the num- 

 ber of specimens is almost infinite ; at low tide miles and miles of flats 

 are covered with them. 



One of the most common species in collections is a little white form 

 belonging to the sub-genus Olivella ; from its resemblance to a grain of 

 rice it has received the specific name oryza. It comes from the West 

 Indies, in some parts of which it occurs in vast numbers. The sub- 

 genus Olivella is distinguished from Oliva proper by the longer spire 

 of the shell and the absence of tentacles and eyes. Olivella biplicata, 

 which is figured, comes from the Pacific coast. 



Of the true Olivas, the most cornmon species in the southern United States is 

 0. littefata, marked with angular markings, which by a stretch of the 

 imagination might be regarded as resembling writing. The general 

 color is a yellowish white, the markings brownish. Another lot of 

 shells are named, from their resemblance to certain rocks, jaspidea, 

 porphyria, etc. The latter species comes from Panama. 



The harp-shells (genus Harpa) differ markedly from the other 

 members of the family by their broad aperture, and swollen, trans- 

 versely-ribbed whorls. Although only nine species are known, every 

 collection contains several specimens, those of Harpa ventricosa being 

 possibly the most common. Large as is the shell, it is not sufiicient 

 to contain the whole animal, and Semper, as well as the older natur- 

 alists, record a peculiar habit of self-mutilation with some of the spe- 

 cies from the eastern seas. When captured, a part of the foot 

 remains outside of the shell. This the animal brings across the sharp 

 edge of the aperture, thus cutting it off. In time a new portion grows out, replacing 

 that which was amputated. The animals are lively, and bright colored. They are 

 found in all the tropical seas, except the Atlantic. Harpa ventricosa comes from the 

 East Indies, H. imperialis from Bourbon, while H. crenata and H. scriba are found 

 at Panama. 



In the MiTRiD^ but a single genus, Mitra, needs mention. Here the shell is thick, 

 long, and fusiform, the spire being well developed and the columella plicate ; the 

 aperture is narrow, notched anteriorly, and in some species is partly closed by a small 

 horny operculum. Mitra episcopalis, possibly the most common member of the genus, 

 is ornamented with spots (usually quadrangular in outline) of red, salmon, or orange. 

 It comes from the Philippine Islands, where it moves rather sluggishly over the flats, 

 especially when the tide has just begun to come in. When the tide recedes, it buries 

 itself just beneath the surface. Some of the smaller species are more lively, and others 

 crawl about on the surface of the sand when the tide is out. Although frequently 

 seen in the day-time, the Mitra-s are essentially nocturnal, and spend most of the hours 

 of daylight hidden under rocks and in holes in the coral reefs. Some species, when 

 irritated, defend themselves by secreting a purple fluid, the odor of which is said to be 

 very nauseating. 



Another eastern species, nearly or quite as common as the one mentioned, is M. 



Fig. ill. — Oliva 

 porphyria. 



