Hyatt.] 



36 [October 5, 



varies exceedingly, as well as the size and prominence of these and 

 the pilae upon which they stand. 



There seems to be one constant difference between this species and 

 M. biferwn. The superior lateral lobes are unequally divided into 

 three minor lobes instead of, as in M. biferum, being equally divided 

 into two. The young septa are precisely similar in development, 

 and also similar to those of the adult and young of Deroceras pla?i- 

 icosta ; the superior lateral lobes being at first equally divided by a 

 cell arising from the side of the superior lateral cell. This is sub- 

 sequently met by a cell advancing from the other side and making 

 the usual threefold division of the lobe. 



Microceras crescens. 



M. crescens Hyatt, Bull. Mus. Com p. 'Zoology, no. 5, p. 82. 



In this species we have a form which is intermediate between M. 

 laticosta and Opliioceras raricostatum. It agrees with the latter in its 

 septa, and with M. laticosta in its pilae and general external char- 

 teristics of form and growth. In one specimen from Rautenberg, 

 there is a Turrillite distortion, but the deviation of form is in this case 

 very marked toward the left instead of the usual dextral twisting. 

 The superior lateral cells are broad and very slightly indented by 

 the minor, divided into two unequal portions, however, by one minor 

 lobe slightly larger than the rest. The superior lateral lobes are 

 equally divided, the inferior laterals very shallow. All the cells are 

 very broad in comparison to the lobes. The superior lateral lobes 

 are about two-thirds as long as the abdominal lobe, and the inferior 

 not more than half as long as the superior lateral. 



Microceras arcigerens. 



Amm. arcigerens Phill., Geol. York, p. 163, pi. 13, fig. 9. 



M. arcigerens Hyatt, Bull. Mus. Comp. Zool., no. 5, p. 82. 



In this species the septa are very peculiar. The outlines are 

 remarkably simple. All the lobes are remarkably broad, the supe- 

 rior laterals and abdominal nearly of the same height, and the infe- 

 rior laterals fully two-thirds as long as the superior .laterals. The 

 whorl is compressed ab domino-dors ally, and much broader on the 

 dorsal than abdominal side. This, and the prominence of the closely 

 set pilse in the young, gives the shell a very decided resemblance to 

 0. raricostatum. It will be observed that in this case the resemblances 

 to 0. raricostatum are in those very external characteristics in which 

 none could be traced in M. crescens. 



