.304 TROCHID2E. 



circular object, which appears to be an operculum; this by 

 some is considered an Annelide I think erroneoiisly 

 as from its texture and sculpture I take it to be the lid of 

 this species. The animal is an important desideratum. The 

 position of Scissurella is little better than a conjecture ; it 

 is a wanderer between Trochus, lanthina and Haliotis. 



We divide this group into two sections, the conically elon- 

 gated and non-umbilicated shells, and the discoidal perforated 

 ones ; merging in the latter the genus Margarita of authors, 

 and the species of the genus Skenea, nonmill., most of the 

 animals of which have been discovered and described by us as 

 Trochi, except the so-called Skenea pkmorbis, which we con- 

 sider a discoidal Rissoa and as there is no sufficient descrip- 

 tion of the animal, one is supplied in the observations on 

 the Rissoa, that naturalists may judge of the propriety of 

 this position. The Skenea nitidissima is almost beyond doubt 

 Philippics Truncatella atomus. The ? S. rota may be a variety 

 of the " atomus," some naturalists think so ; we have at times 

 almost been of opinion that it, or a very similar object, is the 

 terminal coil of the Caecum trachea, on which more will be 

 said hereafter. As for the Skenea serpuloides, which is the 

 " divisa," nonnull., the S. Cutlerianus, mihi, S. leevis, and 

 excepting the very doubtful S. costulata, we have proved them 

 to be Trochidans ; therefore the Skeneadce, a family of past 

 times, having fulfilled its provisional functions, is now only 

 mentioned to account for its old constituents. 



This is perhaps the proper place to relate a curious circum- 

 stance respecting the Trochi, which have hitherto, by Cuvier, 

 and I believe by all naturalists, been considered as bisexual ; 

 which reproductive plan, as regards them and all the genera 

 I have examined that have circular spiral opercula, is shown 

 below to be of very doubtful determination ; and we think it 

 not improbable that, with the genera Ccecum and Turritella, 

 which we have deposited in our new family of the Verme- 

 tidffi, and the freshwater genus Valvata, they will prove con- 

 gressional pectinibranchous hermaphrodites ; and as all have 

 the circular spiral operculum, they will form a very natural, 

 though an aberrant, group of Pectinibranchiata. 



