304 TROCHIDA. 
circular object, which appears to be an operculum; this by 
some is considered an Annelide—I think erroneously — 
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, Ianthina and Halotis. 
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, nonnull., most of the 
animals of which have been discovered and described by us as 
Trochi, except the so-called Skenea planorbis, 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 Rissoe, that naturalists may judge of the propriety of 
this position. The Skenea nitidissima is almost beyond doubt 
Philippi’s 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 Cecum trachea, on which more will be 
said hereafter. As for the Skenea serpuloides, which is the 
“ divisa,’ nonnull., the S. Cutlerianus, mihi, S. levis, and 
excepting the very doubtful S. costulata, we have proved them 
to be Trochidans; therefore the Skeneade, 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 cireum- 
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 Cecum and Turritella, 
which we have deposited in our new family of the Verme- 
tide, 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. 
