A. H. Foord — On the Genus Piloceras. 



543 



in length and 4 lines in its greatest diameter : the largest 4|^ inches 

 in length, and 1| in its greatest diameter. Neither of the specimens 

 measured are perfect. The walls of the siphuncles are usually 

 composed of chalcedony of a minutely concretionary structure.' 



In comparing the structure of Piloceras with that of other 

 Cephalopods, we are at once reminded of Endoceras, and the 

 resemblance between these two forms did not escape Salter's 

 experienced eye. In both genera the siphuncle is very large in 

 proportion to the shell, and in both it is furnished with a series of 

 conical or funnel-shaped sheaths, which apparently communicate 

 with one another by means of a small central tube, the "endosiphon" 

 of Hyatt (Figs. 2, 3). This tube is regarded by Hyatt as analogous 

 to that which has long been known in Actinoceras, and science is 

 indebted to that able observer for its discovery in Endoceras." 

 Fig. 2. Fig. 3. 



Fig. 2. — I. Section of siphuncle of Piloceras, sp., sliowing two of the funnel- 

 shaped sheaths, s ; e, the endosiphon, with r, remains of former sheaths ? ; m, matrix 

 filling what remains of the cavity of the siphuncle ; q, quartz infilling. II. Posterior 

 extremity of another siphuncle much eroded, but showing an aperture, a, at the apex 

 by which the endosiphon may have communicated with the initial chamber ; w, wall 

 of the siphuncle, and q, quartz forming the infilling between this and one of the 

 sheaths. III. Transverse section of another siphuncle showing in the centre the 

 endosiphon e, and what appears to be a partition p, representing perhaps the septum 

 or " central shelly plate " of Dawson. (All the figures are of natural size.) 



Fig. 3. — Vertical section of an imperfect siphuncle, with a few of the septa 

 attached, of Piloceras amplum, Dawson, cut in the direction of the shorter diameter of 

 the shell, which was of an elliptical form, judging by that of the siphuncle. s lu, shell- 

 wall ; s c, siphuncular cavity ; s h, sheath ; tv, internal wall of siphuncle ; «, s, 

 septa ; b, broken extremity of siphuncle ; r and e, same as in Fig. 2. The dotted 

 lines are restorations. (Slightly reduced from Dawson's figure, which is itself a 

 little smaller than the original specimen.) 



^ This form of chalcedony used to be called beekite, but it differs so slightly in 

 composition from ordinary chalcedony that that name is no longer employed for it 

 by mineralogists. 



"^ Proc. Eoston Soc. Nat. Hist. vol. xxii. April 4, 1883, p. 261. 



