OTHER SPECIES OF PLATYCERAS 179 



nian Diaphorostoma and Platyceras, the question arises whether the so- 

 called genus Platyceras may not be of polyphyletic origin. 



PL A TYCERAS THETIS HALL 



The specimens referred to this species have a very short Diaphoros- 

 toma stage, and the coiled apex is free from the main body whorl. The 

 surface is characterized by faint longitudinal folds, which are most reg- 

 ular on the left side of the dorsum. There is a strong sinuosity indi- 

 cated by the growth lines on the dorsum or a little to the right of it. 

 There is also a strong fold on the left lower side which produces a pro- 

 nounced emargination in the peristome. The aperture makes an angle 

 of about 45 degrees with the plane of the whorl. The specimens were 

 found at the base of the cliff at Bartlett's mills and probably belong to 

 the Lower Hamilton. 



PL A TYCERAS B UCCULENTUM HALL 



In the lowest beds at Bartlett's mills a number of specimens were 

 found having the form, obliquity of aperture, and coil of this species, 

 but without the strongly marked fold on the right side. The apex is 

 about as much enrolled as in P. thetis, and the Diaphorostoma stage is 

 about the same length. At first the enlargement is very gentle, then it 

 is more rapid, thus giving the upper portion of the shell a pinched ap- 

 pearance. The plane of the coil is nearly in line with the right margin 

 of the peristome, almost the whole of which lies to the left of the plane 

 drawn through the initial coil. In the largest of the specimens found 

 the characteristic fold is slightly developed on the right side, and the 

 lines of growth are slightly sinuous, there being an addition of several 

 minor revolving folds. In general, however, the species as here repre- 

 sented is without the fold, and the peristome has a regular outline. It 

 agrees very closely with the individual figured by Hall * from Canan- 

 daigua lake, New York, and also the specimen from the Hamilton of 

 the Genesee valley. 



PLATYCERAS QUINQUESINUATUM ULRICH 



A single specimen agreeing in general with the description of this spe- 

 cies, but more slender than those figured by Whiteaves, was found in 

 the coral layer of Bartlett's mills. 



Table showing Distribution of Species in the Thedford Region 



We have recorded in the table the horizon of all the species obtained 

 by us at Thedford and vicinity, and have given as near as possible those 



♦Paleontology of New York, vol. 5, part 2, plate 3, figure 29. 



