84 AMERICAN JOURNAL 



Edentates and the Cetaceans among Mammals. For exam- 

 ple, ^olis and Valuta have not more than a single row of 

 teeth, whilst those of the groups the nearest approaching the 

 tirst genus have numerous rows, and those approximating to 

 the second have three rows; only the teeth which remain in 

 the above genera, resemble the correspond ng teeth in the 

 allied forms. 



Many genera are deprived of teeth, although the neighbor- 

 ing generic groups possess them (for example, Thetys fimbriata. 

 Doridopsis (Hancock), Stilifer, Leptoconchus): but it is possible 

 that these genera possess them in a young and larval state. 

 .The genera Solarium and Terebra, which have been cited as 

 deprived of teeth, really possess them, a result of more exact 

 researches recently undertaken. 



Series I.— MONOTOCAKDIA. 



Heart bilocular, with only one auricle, copulation effected 

 between two individuals by means of the intromission of an 

 organ. 



Class. I. — Androgyna (Musioglossata, Morch, olim.) 



Sexual organs united, always furnished with a receptaculum 

 .5 'minis (petiolate bladder); male organ retractile. 



Tongue provided with multiserial teeth (except JEolis and 

 the Pellibranchs) ; mouth with mandibles. (But this is not so 

 with the Agnatha.)* 



1. Pulmonata. — -P. terrestria, P. fluviatilia (Lymnsea, Pla- 



norbis, etc.), P. marina (Siphonaria.) 



2. Opisihobranchia. — (Gymnobranchs, Tectibranchs, Pelli- 



branchs.) 

 5. Pelagica. — (gymnosomous Pteropods.) 



Class II. — Exophallia. 



Sexes separate; male organ not retractile, sometimes con- 

 cealed in the branchial cavity, or with the tentaculum. Mouth 

 supplied by suction. 



I. Tvcniogtossata. — Tongue with seven rows of sharp, curved 

 teeth. 

 A. Bostrifera. — Mouth at the extremity of a non-retractile 

 muzzle. 



Metamorphosis in the interior of the egg. 



a. Terrestria. Cychstomacea. 



b. Fluviatilia. Paludina. Ampullaria. 



•'■ A group established by M. Morch in 1S59, including Tertacella, D>>>/</' 

 Streptaxu, etc. 



