GENUS TROCHOCERAS. 361 



TROCHOCERAS (Gyroceras) BANNISTERI. 



PLATE 20 A, FIG. 2. /^^-^^ ^ Tx^ J^Ue^J^eyh 



Gyroceras bannisteri [?] : Winchell and Maroy, Meiiioiis of the Boston Society of 

 Natural History, vol. 1, page 102. 



Shell consisting of about one and a half volutions, which increase in size 

 somewhat rapidly : spire gently ascending; umbilicus broad and com- 

 paratively deep, the lateral diameter of the volutions being greater than 

 the dorso-ventral diameter. Surface marked by low subangular annula- 

 tioiis, which, turning gently backwards on the sides of the shell, arc more 

 abruptly bent into a shallow sinus on the dorsum. These annulations 

 apparently become obsolete towards the aperture : on the other parts of 

 the shell, there are about six or seven in a space equal to the dorso- 

 ventral diameter. 

 This species differs from either of the species described ; in its more 



rapid enlargement from the apex, and in the finer annulations. 



These observations are made upon a cast in gutta-percha, sent to me by 



Prof. WiNCHELL under the name of Gyroceras bannisteri. The cast has 



the appearance of a very pretty species of Trochoceras, of more delicate 



proportions than those described in this paper. 



For7natio7i and locality. In limestone of the age of the Niagara group, 



at Bridgeport, Illinois. 



Note. — Since it sometimes happens that tlie Thochoceras desplainense is 

 quite flat upon the upper side, or that the inner volutions are a little 

 depressed below the outer one, it might perhaps be suspected tliat the 

 T. costatum, of which the umbilical side is represented in the figure, is 

 an exaggerated condition of the former species, with the inner volution still 

 more depressed. This, however, is not true, for the volutions are clearly 

 sinistral, and as represented are in their natural relations. It is per- 

 haps the first American species of this character that has been recognized, 

 but M. Barrande distinctly describes the two forms, and has figured 

 several species with sinistral spires. In his generic description, he says 

 " the shell consists of several turns of the spire or of a single volution, 

 "more or less complete, but exhibiting almost always a marked defect of 

 " symmetry. The enrolment is sometimes dextral and sometimes sinistral 

 " according to the species, but the dextral forms greatly predominate. 

 " In Troch. asperum we find both modes, varying in individuals, and it is 

 "the same in Troch. sandhergeri.'''' 



[Arisem. No. 239.] 46 



