376 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 1, 



PiLocEEAs *, gen. nov. 



A broad, conical, and slightly curved shell, subcylindrical (or 

 compressed). Siphuncle and septa combined as a series of conical 

 concave septa, which fit into each other sheathwise. 



PiLOCEEAS rsrvAGiNATUM, spcc. nov. 



PI. XIII. figs. 17-21 ; and Woodcut, fig. 7, c. 

 P. 5 uncias longum, conicum, subcurvum, ore ovali, (long, oris ad lat. utll-8). 



This singular shell, which seems to offer the simplest of aU Ce- 

 phalopodous forms (and to combine in its septal plates both septum 

 and siphuncle), is not unfrequent in the Durness limestone, in a 

 silicified state. None of the several specimens collected by Mr. 

 Peach are perfect. 



The mouth is oval, with the longer diameter about 2 inches, and 

 the breadth 1^ inch, in specimens which would measure above 4 

 inches long. The general form is gently curved, ending in a some- 

 what obtuse point (fig. 19), near which the tube is annulated rather 

 strongly, while in the older portions it is more faintly so. 



The septa, at a diameter of l^- inch, are rather more than one ith 

 of an inch apart, and viewed in section (fig. 21) show only three con- 

 centric sheaths. There is clear evidence only of four septa in the 

 longitudinal section (fig. 20), the remaining lines being due to the 

 crystallized linings of the chambers. These septa end in a sharp 

 point downwards, the walls meeting at an angle of about 35°, or less. 



Comparison is naturally sought for this remarkable genus among 

 the numerous Cephalopods of the same zone. In America the Or- 

 thocerata with large lateral siphuncles (woodcut, fig. 7, a) abound, 



Fig. 7. — Diagram, of some forms of Lower Silurian Orthoceratidce. 



a. Cameroceras. h. Endoceras. c. Piloceras. 



and in some of these the diameter of the siphuncle is so great in 

 proportion to the size of the shell, as almost to realize the form now 

 under consideration. Endoceras proteiforme, Hall, Pal. New York, 

 vol. i. pi. 48, &c., and still more E. magniventrum, pi. 53, show not 

 only a great siphuncular space, but also a pointed termination of the 

 siphuncle itself (fig. 7, h), which thus appears as an elongated lobe or 

 process of the hinder part of the body, hke one of the many lobes of 

 * From TTtXos, a cap, and the usual termination Kepas. 



