PALAEONTOLOGY OF NEW YORK. 



transverse stria?, crossed by finer longitudinal lines. The walls of the air- 

 chambers are flat, and the septal sutures but slightly impressed. 



A fragment, embracing fifteen mm. of the length of the chamber of habi- 

 tation, and fourteen of the attached air-chambers, has a length of fifty-three 

 mm. A septate portion of a larger individual has eleven air-chambers in 

 the length of thirty-five mm. 



This form is distinguished by its transverse section, the peculiarities of the 

 septa, and the size and position of the siphuncle. The specimens present an 

 unusually compact and smooth appearance, owing to the numerous and regular 

 septa. 



Formation and locality. From the coarse, calcareous beds of the Hamilton 

 group at Pratt's Falls, Onondaga county, N. Y. 



Orthoceras constrictum. 



PLATES I.XXXIV, FIGS. 13, 14, 16; LXXXV, FIGS. 5, 10, 11, 13. 



Orthoceras constrictum, Vakvxbm. Qeotog. Surv. of N. Y. : Rep. Third Dist., p. 152, fig. 1. 1S4-.'. 

 •' " (Vanuxem), Hall. Descriptions of New Species of Fossils, etc., p. 49. 1861. 



Fifteenth Rep. State Cab. Nat. Hist., p. 77. 1868. 

 Not - " Conrad. G«olo#. Survey of N. Y. : Pal. Dept., Annual Rep., p. 117. 1838. 



Shell straight, regularly enlarging from the apex to the chamber of habitation. 

 Transverse section circular. Apical angle about 6°. Initial point unknown. 



Chamber of habitation cylindrical, well developed, having a length equal 

 to four times the diameter at the last septum. Anterior to the middle there 

 is a very broad, gentle constriction, which, in its position and strength, varies 

 considerably, sometimes becoming a very conspicuous feature. Air-cham- 

 bers numerous, regular, with a depth of from two to three mm. 



Septa smooth, thin ; the concavity equal to an arc of nearly 90°. Sutures 

 straight and horizontal. Siphuncle central. Its elements in the interseptal 

 spaces have not been observed. 



Test not preserved. Some of the casts exhibit traces of lamellose stria? 

 of growth. 



