264 PALEONTOLOGY OF NEW YORK. 



The largest fragment referred to this species has a length of 100 mm., and 

 is septate throughout its entire length, of thirty-three chambers. A portion 

 of a small individual shows nineteen annulations in the length of seventeen 

 mm. Another example, preserving the chamber of habitation, has thirty- 

 six annulations in the space of seventy-five mm. 



This species closely resembles 0. Thoas, but may be distinguished by its more 

 rapidly enlarging tube, and its much more frequent annulations and septa. 

 Formation and locality. In the Schoharie grit at Schoharie, N. Y. 



ORTHOCERA.TA OF THE UPPER HELDERBERG LIMESTONES. 



In the limestones above the Schoharie grit, included in the Upper Helder- 

 berg group proper, the variety of forms in this genus is very limited. This is 

 in part due to the paucity of material : the whole number of individuals 

 observed, among collections made during a series of years, not amounting to 

 more than fifty fragmentary examples of the genus. The number of species 

 recognized bears a large ratio to the actual number of individuals observed. 

 The condition of preservation of the specific characters is, however, in general 

 better than in the Schoharie grit; but the extremely few examples and 

 fragmentary mode of occurrence, render any attempt at a satisfactory determina- 

 tion of the entire characters impossible. The species here described are only 

 those presenting decided marks of distinction from the established forms. 



Appended below is a list of the species occurring in the limestone, which have 

 been doubtfully identified with species occurring in other geological horizons : 



Schoharie Grit. 

 Ortkoceras Pelopg, Hall. See page 233, under description of 0. Pelojis. 



" tetricum, " Fragments have been observed having the dimensions, apical angle, and depth 



of air-chambers of this species. 

 " obliquum, " (= 0. vaxtator, H.)* A specimen having the general aspect and proportions Of 



the individuals referred to this Bpecies. 

 " Thoas, " Noticed under the description of 0. Thoas, and in the explanation of plate 41. 

 " Creon, " A fragment of the test referred to this species, and noticed in the description. 



* See explanation of plate 78, and corrections. 



