PTEROPODA. 261 



The T. arenosus has only been found in a single locality in the Helderberg 

 range in the town of Knox, Albany county, N. Y. 



The T. scalariformis is rare at Schoharie, and has been found in two localities 

 in central and western New York. It seems to be the nearest representative 

 of the T. scalaris of Europe which we have in this country, judging from all the 

 figures of that species accessible to me, and it may be only a geographical 

 variety. The form indicated as T. sicula is identical with T. scalariformis, so 

 far as can be determined from the condition of the New York specimens, 

 and, while rare in this State, is extremely abundant in the neighborhood of 

 Sandusky and Delaware, Ohio. It also occurs in Charlestown and Pendleton, 

 Indiana. 



The T. attenuatus is very abundant in some decomposing arenaceous shales 

 in Otsego county, N. Y., and T. bellulus occurs in several localities in the central 

 part of the State ; but neither of them are known to me as occurring in the 

 western counties, or in any localities of the Hamilton group beyond the limits 

 of New York in a westerly direction. On the other hand, the T. attenuatus is 

 extremely abundant in some arenaceous beds of the Hamilton group at 

 Saddleback Ridge, in Huntington county, Pa. ; a point at least three hundred 

 miles southward from the localities in the State of New York. 



The T. spiculus of the Chemung group is very numerous in certain decom- 

 posing layers to the south of Ithaca, and in Cortland county, N. Y. ; but no speci- 

 mens have occurred in all the extensive collections made in the country west 

 of the Genesee river, nor in all the southern range of counties of the State. 



The Devonian species, T. Hoyti of White, and T. subtilis, described by 

 Winchell, we know only from the localities cited by their authors. 



Of the species from the rocks of Lower Silurian age, we have no other 

 knowledge of their geographical distribution than as given by their authors, 

 who cite the occurrence of the species in three localities in the State of Illinois 

 and one in Indiana. 



From our present knowledge, therefore, we might conclude that the distri- 

 bution of the species is very unequal; but it is more probable that our 

 collections are too incomplete to admit of any satisfactory generalization at 

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