PTEROPODA. 171 



or more fine, regular, transverse striae — the usual number on the larger 

 part of the tube being four or five, but sometimes increased, in exception- 

 ally wide spaces, on some individuals, to eight. 



As a rule, the annulations in this species increase in distance towards the 

 aperture, but they are for the most part placed at irregular intervals, the one 

 figured being the most regular and uniform of any observed. 



Measurements of several individuals, where the impressions are sharply 

 preserved in the stone; show about eight or nine annulations in the space of 

 five mm. from the larger extremity; and in an equal distance beyond, there 

 are twelve or fourteen or more. In these specimens the annulations are 

 sharply defined, narrow and abruptly elevated from the body of the shell, and 

 without evidence of striae. 



The entire length of this fossil is from ten to twelve mm., and rarely a little 

 more. It occurs as casts and as impressions of the exterior shell in argilla- 

 ceous sandstone, and is not at this time known in any other condition. 



In general character and in details of surface-marking, this species is very 

 similar to T. bcllulus ; it differs in being of smaller size, and in the irregularity 

 in distance of the annulations. This fossil occurs in myriads, occupying thin 

 layers in the argillaceous sandstone, and is never of greater size and length than 

 represented in figure 19 of plate 31. (Fig. 20 is enlarged to five diameters.) 

 The T. ht'lltihis occurs but rarely in the calcareous shales in the central part of 

 the State ; and if we suppose the T. attenuatus to be only a smaller variety of 

 that form, we have evidence that the conditions required for individual 

 development have not existed at the period of the argillaceous sandstone, since 

 a piece of the rock of one cubic inch, holding the T. attenuatus, contains a 

 greater number of specimens than have ever been seen of T. bellulus in all the 

 collections. 



Formation and localities. In thin bands of shaly sandstone near Cooj>erstown, 

 at East Worcester, and other places in Otsego county, N. Y. The same form 

 occurs in some thin-bedded, impure sandstones and coarser beds, associated 

 with Pit ri nca flabellum and Homalonotus Dekayi, at Saddleback Ridge in Hunt- 

 ingdon county, Pennsylvania. It likewise occurs in the shales of the Hamilton 

 group at Arkona (C. W.) Ontario, on the authority of Dr. Romingek. 



