324 LOWER INVERTEBRATES. 



closed behind as growth progresses, and the result is that each whorl receives a band, 

 which is quite distinct from the rest of the shell. In some of the fossil species this is 

 very marked, and stands up , elevated to a considerable distance beyond the sur- 

 rounding surface ; in others the closing up is not complete, and the result is that there 

 remains a series of holes like those of the abalone {Haliotis). Recent deep-sea ex- 

 plorations have largely increased the number of known species of this family, most of 

 which are apparently inhabitants of water from four hundred to a thousand fathoms 

 in depth. 



The Helicinid^ is a family of terrestrial gasteropods variously placed by different 

 naturalists. Most commonly the members are placed among the true land shells (pul- 

 monates), but with these they have little or no affinity. We follow 

 Claus in assigning them their present position. In appearance of 

 the shell, in the structure of the lingual ribbon, as well as in their 

 habits, they are much like Hdix, living as they do upon the land, 

 either concealed under the dead leaves on the ground, or among the 

 ^^"'rarieo^o.'""" branches or foliage of the trees. They differ however, from the 

 Helicidfe, among other important points, in the possession of an oper- 

 culum. The aperture of the shell is semilunar, and the umbilicus is covered by a 

 callus. The species are all tropical, and are mostly confined to the American con- 

 tinent and the West India Islands. Only a few species out of the five hundred 

 known are found within the limits of the United States. The prominent genera are 

 Selicina, Stoastoma, and Proserpina. 



Oeder V. — CTENOBRANCHIA. 



Most of the members of this group despise a vegetable diet and prefer to live on 

 animal matter whether living or dead. Still, some exceptions occur which will be no- 

 ticed in the proper places. In all, the shell is spiral, and the gill of the normally right 

 side is alone present. What has previously been considered as the rudimentary gill of 

 the left side has been shown by Spengel to be the highly developed olfactory organ. 

 The gills have a comb-like shape and the axis is frequently attached to the roof 

 of the branchial chamber, which by that torsion of the body described on a preceding 

 page, is brought above the neck of the animal. In many, a well-developed copulatory 

 organ is found on the right side of the neck, and the proboscis may or may not be re- 

 tractile. The former condition of affairs has given rise to a group called Proboscidi- 

 fera, containing the families Tritonidse, Doliidae, and part of the Mnricidse as here lim- 

 ited, while another group, containing the Cypraeidse, Velutinidse, and Naticidse, have 

 the rostrum invertible only at the tip. 



The Ctenobranchia is divided into four sub-orders, the distinctions being largely 

 founded upon the arrangement of the teeth upon the lingual ribbon, although other 

 characters are of course employed. 



Sub-Ordek I. — Ptenoglossa. 



In this group the shell has the aperture entire ; that is, the whorls are complete and 

 the edges of the aperture are not notched or prolonged into canals. No respiratory 

 siphon is formed, and copulatory organs are absent. The tongue is armed with numer- 

 ous small teeth on either side, but lacks the normal middle row. 



