PTEROFODA. 
m 
or more fine, regular, transverse stake—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 strise. 
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. bellulus; 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 diameter's.) 
The T. bellulus 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 slialy sandstone near Cooperstown, 
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 Pterinea Jiabellum 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. Rominger. 
