CEPHALOPODA. 101 



opening for the respiratory funnel being nearly distinct from 

 the cephalic aperture. In Cyrtoceras the curvature is dorsal. 

 In some other members of this family the siphuncle 

 attains a remarkable size or extraordinary complexity. In 

 Camaroceras (fig. 30, 4), the siphuncle is lateral, quite simple, 

 and equal to half the diameter of the shell. Casts of these 

 great siphuncles were called "Hyolites" byEichwald; they 

 frequently contain small shells of Orthoceras, Bellerophon, and 

 other genera. In some species the siphuncle is strengthened 

 internally by repeated layers of shell, or partitioned off by a 

 succession of funnel-shaped diaphragms ; these constitute the 

 genus Endoceras of Hall. The same author has given the 

 name Discosorus to a fossil which is evidently the siphuncle 

 of some very delicate and perishable chambered shell (fig. 2, 6). 

 In those Orthocerata with siphuncles most nearly resembling 

 the Discosorus they diminish rapidly towards the last chamber. 

 Perhaps the most remarkable fossil of this group is the Hu- 

 ronia (fig. 30, 5), found in the upper Silurian limestone of 

 Drummond Island. Siphuncles 6 feet in length and 1 \ inch 

 in diameter, stand out in bold relief from the cliffs ; they are 

 silicified, and are unaccompanied by any vestige of the shell, 

 except in one or two instances, where the septa are faintly 

 indicated by coloured lines. They are sometimes overgrown 

 with coral, and were evidently so durable as to remain on the 

 sea-bed long after the shell itself had decayed. The joints of 

 the siphuncle are swollen at the upper part, and the interior 

 is filled with an irregularly-radiated structure, apparently 

 produced by the plaiting and calcification of the lining 

 membrane. This structure also exists and is very regular 

 in the siphuncle of the Devoniaa Orthoceras trigonale, in the 

 shells referred to Gyroceras by d'Orbigny (fig. 30, 8) and 

 in Actinoceras, a sub-genus of Orthoceras, discovered by Dr. 

 Bigsby, and described by Stokes.* The plication of this 



* Geol. Trans., vol. i., 1825. 



