84 AMERICAN JOURNAL 



Edentates and the Cetaceans among Mammals. For exam- 

 ple, jS^joUs and Voluta have not more than a single row of 

 teeth, whilst those of the groups the nearest approaching the 

 lirst 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, Thetysfimhriata, 

 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.— MONOTOCARDIA. 



Heart bilocular, with only one auricle, copulation ejft'ected 

 between two individuals by means of the intromission of an 

 <:)rgan. 



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



Sexual organs united, always furnished with a receptaculum 

 seminis (petiolate bladder); male organ retractile. 



Tongue provided with multiserial teeth (except jEoUs and 

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

 with the Agnatha.)* 



1. Pulmonata. — -P. terrestria, P. fluviatilia [Lymnvea, Pla- 



norbis^ etc.), P. marina (jSij^honaria.) 



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



branchs.) 

 5. Pelagica. — (gymnosomous Pteropods.) 



Class II. — ExopnALLiA. 



Sexes separate ; male organ not retractile, sometimes con- 

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

 supplied by suction. 



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

 teeth. 

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

 muzzle. 



Metamorphosis in the interior of the egg. 

 n. Terrestria. Gyclostomacca. ' 

 h. Fluviatilia. Paludina^ Ampnllitrid. 



* A group establinbcd l>y M. Morch ii> 1859, includii>g Tfistar.elhi, J>au.dn- 

 hardia, StreptaxU, etc. 



