PTEROPODA. 201 



Hyolithes triliratus, n. sp. 



PLATE XXXII A, FIGS. 20-22. 



Form an elongate triangular pyramid, with a scarcely perceptible curvature. 

 Transverse section subtriangular, with a width nearly twice as great as 

 the height. Ventral face moderately convex, transversely and distinctly 

 curving in a longitudinal direction. Dorsal side convex, scarcely angular 

 along the centre, with the two sloping faces gently curved ; the median 

 line marked by a distinct longitudinal groove (one specimen having a 

 slender carina along its centre) ; and the sloping sides with a narrow groove 

 nearly parallel to the margins, and about one-fourth the distance from 

 the margins to the median groove. Aperture undetermined, and the 

 ventral and dorsal borders unknown — the two specimens known being 

 imperfect. 



Surface originally marked by extremely fine, longitudinal and concentric strije, 

 which, in the specimens examined, are obscurely visible under a good lens. 



The specimens are either casts of the interior, or of the exterior surface 

 where the shell has been extremely macerated, with scarcely more than a thin 

 film remaining. 



The length of this species, in the longest individual, is about twenty millime- 

 tres, with a width of seven or eight millimetres. 



The form of this fossil is similar to H. aclis in its upper part, but it does 

 not expand so rapidly towards the aperture. The summit on the dorsal side 

 is more rounded, without the angularity and tendency to carination sometimes 

 observed in that species. The surface-markings are more subtle, or less dis- 

 tinctly preserved under similar conditions. The form is less attenuate than 

 //. strintus. 



Formation and localities. In the coarse arenaceous shales of the Hamilton 

 group on Sherburne creek, Chenango county, and in the argillaceous shales 

 of the same formation on the east side of Cayuga lake, near Norton's 

 Landing, N. Y. 



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