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, Huronia, 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 Avhich may be referred to for addi¬ 
tional features illustrating various phases of the organic deposit. On plate 
415 he figures a species (0. arcitcnens 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. 
From 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. 
