PURPLE-SNAILS, WENTLE-TRAPS, ETC. 633 



moreover, are monopectinate ; that is to say, the gill has only one row of 

 filaments. 



The Pectinibranchiata are further divided into sub-orders according to the 

 characters of the radula. 



Sub-order 1. GYMNOGLOSSA. Under this are grouped some more or less 

 parasitic Mollusca which have no radula. The arrangement is artificial but 

 convenient. Two families are distinguished, both of universal distribu- 

 tion : EULIMID.E and PYRAMIDELLID/E. The former are mostly high-spired, 

 very glossy ; small white shells, with slightly bent axis ; at times furnished 

 with an operculum. S >me species are found crawling in the stomachs of 

 sea-cucumbers (Hulothuria), others affix themselves to the exterior of the 

 same creatures by means of the long proboscis, which is thrust through the 

 skin of the host. The genus Stylifer has a glassy, globular shell, and lives 

 amongst the spines of Echinoderms or buried in the skin of star-fish. The 

 Pyramidellidse form the preceding family by the occurrence of plaits or folds 

 on the columellar, and by the fact that the protoconch is sinistral, whilst the 

 rest of the shell is dextral and the axis of the former inclined at a consider- 

 able angle to that of the latter. 



Sub-order 2. PTENOGLOSSA. Two families are associated, probably pro- 

 visionally, under this sub-order, having but one feature in common, viz., the 

 radula contains a great number of similar teeth in each row, the largest 

 being at the outer edges. 



The IANTHINID/E, or purple sea-snails, float about in mid-ocean. Their 

 violet shells are thin and translucent ; the animal has a distinct snout ; the 

 eyes are generally wanting. Two plume-like gills are present. At certain 

 seasons a membranous float or raft, full of air cells, is secreted by the 

 animals, the egg-sacs are attached beneath, and when the whole is complete 

 it is cast adrift. They seem to be gregarious, and stray parties brought by 

 the Gulf Stream are frequently stranded on our western shores. 



The SCALARIID^E, or wentle-traps, are high-spired shells, with the whorls 

 often only united by the prominent varices that occur at regular intervals 

 and impart to the shell its most characteristic feature. The aperture is 

 circular, the operculum horny and spiral. The animal has a retractile 

 snout. The Scalariidse are carnivorous snails and widely distributed. Four 

 species are British. 



Sub-order 3. T^ENIO^LOSSA. To this the greater part of the Pectini- 

 branchiata belongs. The radula usually has the formula 2'1*1'1'2. The 

 various families may be conveniently taken in the following order, bub the 

 position in the group of the first two ia still undecided. 



The SOLARIID/E, called "stair-case shells," from the appearance of the 

 whorls as seen within the open umbilicus of Solarium, comprise forms having, 

 for the most part, depressed shells with wide and deep umbilicus. The 

 opercula vary in the different genera. 



The HOMALOGYRIDJE include one genus Homaloyyra which has a very 

 small, flat, coiled shell with circular operculum. The animal has no tentacles, 

 and the eyes are not on stilks ; the front of the foot is lobed and covered 

 with fine hairs. 



The NATICID^ have, for the most part, very stout, globular shells in which 

 the umbilicus is sometimes open, sometimes more or less completely filled 



