Contributions to Invertebrate Palaeontology. 527 



an inch at the lower extremity. Section circular ; tube moderately increasing 

 in diameter with increased length, slightly curved throughout, and marked by 

 regular encircling annulations, which are elevated, round on the crest, sepa- 

 rated by deeply concave interspaces, which regularly increase in distance and 

 also in strength from below upwards. Those of the lower part where the shell 

 is uncompressed and is half an inch in diameter, are about one-tenth of an inch 

 distant from each other ; and at the upper end where the diameter is about one 

 and three-fourths of an inch are about three-eighths of an inch from crest to 

 crest. Surface of the shell marked by fine, closely arranged and sharply ele- 

 vated concentric striae, and also by longitudinal strise of similar character, but 

 more or less alternating in strength, the two sets giving a finely cancellated 

 structure just discernable to the unassisted eye. Septa very deeply concave 

 and regularly curved, uniting with the shell a little above the crest of each 

 aunulation. Siphuncle small and centrally situated. 



The species is of the ordinary annulated type differing from other 

 species of the group onl}" in the strength and comparative distance 

 of the annulations ; in the rate of increase in diameter, and in the 

 nature of the surface markings. The shell, like many of the annu- 

 lated forms of any considerable size from the Upper Helderberg 

 and Hamilton groups, shows a slight curvature of the tube, a little 

 more perceptible in the lower part than above. The Ohio specimens 

 correspond more nearly to the one from the Hamilton group of 

 N. Y., figured by Prof. Hall (Pal. N. Y., Illust. Dev. Foss., PL 43, 

 fig. 14), in the rate of increase in the diameter, and in the form and 

 relative strength of the annulations than with the original speci- 

 mens to which the name was first applied, or to most of those 

 figured under the same name on the same plate. The specimen from 

 Ohio figured on PI. 41, fig. 9, Illust. Dev. Foss.. Pal. N. Y., under 

 the name 0. Thoas, is identical with the one here described, but 

 does not retain the shell nor show surface markings, but corresponds 

 in the form of the annulations and in its slight curvature and rate 

 of increase in diameter, in which particulars it differs materially 

 from those from New York, given on the same plate. It is barely 

 possible the Ohio specimens may represent a species distinct from 

 any of those from New York, but it seems totally impossible to de- 

 tect characters sufficient to distinguish it as such. 0. subulatum 

 Hall, from the Marcellus shell is a very closely allied if not identical 

 form. 



Formation and Locality. — In the cherty layers of the Upper 

 Helderberg group, near Dublin ; and in the limestone of the same 

 formation near Delaware and Columbus, Ohio. 



