544 A. H. Foord — On the Oeniis Piloceras. 



Professor Hyatt, whose conclusions regarding tlie present genus 

 are based upon Principal Dawson's paper, explains the origin of the 

 sheaths in Piloceras to be due to the widening of what he terms the 

 " fleshy siphuncle " near the body-chamber. The sheaths lie some- 

 what loosely in the siphuncle, and they are supposed by Dawson, 

 upon the evidence of the specimen described by him (Fig. 3), to 

 have existed only temporarily, and to have been successively absorbed, 

 the last one only becoming completely calcified. In the Durness 

 specimens, however, it is not an uncommon thing to find two or 

 three of the sheaths preserved, their walls standing out in relief 

 from the surrounding matrix, which has been removed by weathering. 



A series of short, upwardly projecting lines are observed to spring 

 from the sides of the endosiphon (Figs. 2 and 3), where it is slightly 

 swollen. These lines are conjectured by Dawson to be the remains 

 of the membranous or fleshy sheaths which have become absorbed, 

 as suggested by Hyatt. The close correspondence of these structures 

 in the Scotch and Canadian species is very apparent in Figs. 2 and 3. 

 It is also noticeable that above the sheath whose apex is perforated 

 by the endosiphon there is another one, in a somewhat shrunken 

 condition ; this appeai-s also to be perforated, presumably for the 

 passage of the endosiphon, if indeed that organ extended beyond the 

 apex of the larger sheath below, but of this I have no proof, as no 

 vestige of it can be seen above where it is figured. There is every 

 reason to suppose, though I am not able to demonstrate the fact, that 

 the endosiphon opened into the initial chamber, or at least into one 

 of the earlier of the septal chambers, since in all specimens in which 

 the apical extremity of the siphuncle is preserved, it is found to be 

 perforated. 



It should be mentioned that Principal Dawson in his description 

 of P. amplum refers to the endosiphon as a "vertical partition" 

 crossing the lower part of the siphuncle ; but Hyatt, recognizing its 

 tubular character in the figure, interpreted it in the manner already 

 described. Nevertheless there seems to have been an internal septum 

 extending upwards from the lower part of the siphuncle, between 

 the wall of the latter and that of the sheath into which the endosi- 

 phon opens. This septum shows itself in some transverse sections 

 of the siphuncle in the manner indicated at p, Fig. 2, III., and 

 it can be traced for some distance upwards in the vertical section of 

 this and of other specimens. I am unable, however, to give any 

 satisfactory account of it, owing to its imperfect condition. I venture 

 to think that Sir William Dawson, seeing this septum at the broken 

 end of his specimen (Fig. 3, b), concluded that the endosiphon (e, e) 

 above it v^as its upward continuation. 



As regards the infilling of the siphuncle in Piloceras, it seems 

 obvious that the space between the inner wall of the siphuncle and 

 the first permanent sheath was not originally solid, because it is not 

 filled with the matrix, but either with calcareous, or dolomitic, or 

 siliceous matter, introduced by infiltration. Something was there- 

 fore required to give support to the sheaths, and this was supplied, in 

 part at least, by the endosiphon, and perhaps also by the " septum " 

 above mentioned. 



