273 
THE PLATYCERAS GROUP OF PALEOZOIC GAS- 
TEROPODA. 
By CiiAf.i.Es Koi-i.is Keye>', Dei- Moiiie-. 
It has bcfn shown recently* that there are no good grounds for 
separating, generically. the recent forms of Capulus from tlie more 
typical Platycerata. Kven if it were desirable to rlistinguish be- 
tween the modern and ancient sections. Conrad s familiar title 
could not stand. For, aside from being synonymous with Mont- 
fort's genus. Platyceras had been preoccupied as a generic term 
for more than three-quarters of a century. The name was used 
by Geoff reyt as early as 1740. for a genus of Coleoptera : and again 
by LatreilleJ in 171h;. Nor can ^Khlert's? recent revival of Phil- 
lip's Acroculia be sustained, since the typical species are also true 
Capuli. 
Besides tlie two genera alread}' mentioned, the shells of this 
group have been variously assigned to Pileopsis of Lamarck,*^ 
Actitaof Fisher von Waldheirn.** Acroculia of Phillips, tt (Jrlhon- 
ychia of Hall,t+ and some others. The relative merits of these 
names, as generic titles, have been fully discussed elsewhere.?? 
There has loii'j hcen consideral>le difficulty in attempting to 
separate certain Uifrnhcrr, of tlic grou[) iu question, on tlie one 
hand from some forms of J^latystoma, especially from those spe- 
cies in which there is a greater or less tendency for the shells to 
uncoil: and. <mi the otlicr iiand. from various genera of patelloid 
shells. As might lie expected in a group of gasteropods present- 
ing so few constant characters, which can be satisfactorily relied 
upon as classificatory criteria, it is often almost impossible to 
clearly distinguish between certain of these species. Many struc- 
tural features heretofore considered important in identification 
liavc been ^liown. recentlx . to possess very little, if ;iny. specific 
value, owing to their great variability. It therefore becomes nec- 
essary to regard as of the utmost significance the basing of species 
♦American* Geolooi.st. Vol. vi, p. 0. 
+Hi8t. abr<^g<''e fJes Insectesi, ClT64j. 
^I'n'cis des characteres des Insectes, (ITOfJj. 
§Bul. Soc. (i<:-ol., France, (3), t. .\r, p. 6U2. 
• Anim. san.s V'ertebr., t. vi, p. 16. ri822.j 
**Mem. Soc. imp. Nat. Moscau, t. vr, p. 234. (1823.) 
++Pal. Foss. Cornwall, p. 93. (1840.) 
t^Iiep. 4th Dibt , New York, p. 172. ( 1843.J 
f^Proc. Acad. Nat. Sd., Phila, p. I."i0-l81. flSftO.) 
