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but it has an open gyroceran spiral, the siphuncle is nearer the 

 venter and the costae are more highly developed and more promi- 

 nent, and have a distinct character from those of that species. 



Plectoceras Bickmoreanus, sp. Whitfield (BulL Am. Mils., 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* n. 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 (Trochi) angulation, 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 wa.s described and figured by Eichwald 

 under the name of Clymenia antiquissima, but was subsequently 

 considered as a distinct genus by Barrande. f 



The genus has been subsequently recognized by Schroder 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 Disc, antiquissimum. Similar species 

 having costations throughout life cannot be included in the same 

 genus with those that have them only in the neanic and earlier 



*ns7<rp.a, a cable. 



JiSyst. Sil. de la Boheme, ii, p. 177. 



