CEPHALOPODA. 247 



satisfactory evidence as to its true elements. In two specimens it has been 

 exposed by the process of weathering, and preserves its moniliform character. 



According to Barrande, the apparent explanation as to the nature and purpose 

 of the organic deposit, is that it was secreted by the animal to give strength 

 and weight to the shell, and that it appears to be the analogy of the large and 

 complicated siphuncles of Endoceras, Hitronia, and Actinoceras. (See Syst. 

 Silurien du centre de la Boheme, vol. ii, Text iv, p. 280.) It has been suggested 

 that the shells of Orthoceras and other related genera were probably carried 

 in a vertical position. The volume of the septate or chambered portion being 

 considerably in excess of the chamber of habitation, and the external shell 

 comparatively thin, a deposit on the interior of the chambers would afford the 

 required strength and gravity. The thinness and delicacy of the septa and 

 chamber walls is very marked in Huronia and Actinoceras, when compared 

 with the size and strength of the siphuncular tube. M. Barrande describes 

 several species of Orthoceras, presenting an organic deposit similar to 0. 

 luxum. (See pi. 227 and pi. 446, of Syst. Silurien du centre, de la Boheme.) Plates 

 228, 229, 282, 404, 482 represent species which may be referred to for addi- 

 tional features illustrating various phases of the jorganic deposit. On plate 

 415 he figures a species (0. arcitenens Barr.), showing a deposit or marking on 

 the ventral side of the chambers, similar to specimens of 0. luxum and several 

 species from the Schoharie grit and Hamilton group. M. Barrande also 

 remarks that this and one other obscure species are the only ones with this 

 peculiar deposit that have come under his observation. 



Prom 0. Tantalus this species is distinguished by its smaller size, its more 

 frequent septa, and the amount and character of the organic deposit. It some- 

 what resembles O. oppletum, but is distinguished by its central siphuncle, the 

 smaller distance between the septa, and the lesser development and different 

 aspect of the organic deposit. 0. rudicula, from the Upper Helderberg limestone 

 at Stafford, N. Y., closely resembles this species in some of its phases. 



In its vertical distribution, this species is confined, so far as known, to the 

 Schoharie grit. In its horizontal distribution it has been observed only in the 

 Helderberg range, and in the vicinity of Schoharie. 



