500 
but it has an open gyroceran spiral, the siphuncle is nearer the 
venter and the cost are more highly developed and more promi- 
nent, and have a distinct character from those of that species. 
PLECTOCERAS BICKMOREANUS, sp. Whitfield (Bu//. Am. Mus., New 
York). 
This species of the Niagara fauna has an open gyroceran whorl, 
and in the gerontic stage the last whorl is free and in some speci- 
mens completely straightened out and lituitean in aspect. 
Peismoceras,* nN. g. 
Under this name I propose to separate such costated forms as 
those described by Barrande as Trochoceras, but which differ from 
true Trochoceras and Discoceras in the development of the form, 
outline of the apertures, position of the siphuncle and so on. 
Peismoceras (Troch.) angulatum, optatum, placidum, disjunctum, 
sp. Barrande and others agree in having plain rims to the apertures 
without deep sinuses, except, of course, the ventral sinus. 
The siphuncles are ventrad of the centre, the whorls oval in sec- 
tion, the volutions are barely in contact or open, the apices are 
very large and the umbilical perforations excessively open and 
large, and there is no impressed zone at any stage. . 
Discoceras. 
The type of this genus was described and figured by Eichwald 
under the name of Clymenia antiguissima, but was subsequently 
considered as a distinct genus by Barrande.+ 
The genus has been subsequently recognized by Schréder and 
Remele, but it has by both these authors been used for the smooth 
forms, having a quadragonal section to the whorl and dorsal 
siphuncle, as well as for the costated shells. : 
The neanic stage of all the smooth shells of allied species having 
the siphuncle dorsal and, therefore, formerly included under the 
same name, has decided costations with the same aspect and con- 
tour as in the adult of Drse. antiguissimum. Similar species 
having costations throughout life cannot be included in the same 
genus with those that have them only in the neanic and earlier 
*[letopa, a cable. 
7Syst. Sil. de la Boheme, ii, p. 177. 
