CEP HA LOP DA. 295 



two mm. where the tube has a diameter of twenty-five mm. In the cavities 

 of the air-chambers its elements have not been observed. 



The test has a thickness of nearly one mm. over the chamber of habita- 

 tion. Surface marked by regular longitudinal ridges, with a single interme- 

 diate smaller ridge between, crossed by fine, sharp, crenulating, concentric 

 striae. 



The internal mould usually preserves the longitudinal furrows and ridges 

 without the concentric stria}, but they are less prominent than on the exterior 

 surface of the test. 



One fragment, consisting of the chamber of habitation and several attached 

 air-chambers, has a length of 135 mm., with a diameter of nearly fifty mm. 

 at the larger extremity. Other smaller fragments have been observed 

 which do not furnish any additional data as to the dimensions of the shell. 



This species is distinguished from 0. profundum of the Upper Helderberg 

 group, by the finer concentric striae and the intermediate smaller longitudinal 

 ridges between the prominent ones; and from O. JEgea by the absence of the 

 low annulations of the tube and the numerous and irregular longitudinal ridges 

 and striae of that species. 



Formation and localities. In the shales of the Hamilton group, at York, Gen- 

 eseo, Avon and other localities in the western portion of the State. 



Orthoceras JEgea. 



PLATE LXXXII, FIGS. 7-1S. 



OrtJinccrns JEyra, Hall. Descriptions of New Species of Fossils, etc., p. 52. 1861. 

 Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 80. 1862. 



Shell straight, regularly and somewhat rapidly enlarging. Transverse section 

 very broadly elliptical. Apical angle about 10°. Initial extremity unknown. 



The extent of the chamber of habitation has not been observed. Air- 

 chambers regular, having a depth of about seven mm. 



Septa smooth, with a concavity equal to an arc of 114°. Sutures straight 

 and horizontal. 



