160 
NAUTILOIDEA. 
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 the figures. 
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 appears 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. 
It should be mentioned that Principal Dawson in his description 
of P. amplum speaks of a “ vertical partition ” crossing the lower 
part of the siphuncle ; but Hyatt, recognizing its tubular character 
in Dawson’s figure, refers to it as the endosiphon. 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 endosiphon opens. This septum 
shows itself in some transverse sections of the siphuncle in the 
manner indicated at fig. 17, III., and it can be traced for some 
distance upwards in the vertical section of this and of other speci- 
mens. The septum seems to have been penetrated by the endo- 
siphon, as shown in the figure, but I am unable to give any satis- 
factory account of it, owing to its imperfect condition. 
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. 
Whatever the functions of the endosiphon may have been, I do 
not imagine that it played such an important part in the vital 
economy of the animal as did the same organ in Actinoceras. 
Piloceras invaginatum, Salter. 
1859. Piloceras invaginatum, Salter, Quart. Joum. Geol. Soc. vol. xv. 
p. 376, pi. xiii. tf. 17-21, and woodcut, f. 7 c. 
1877. Piloceras invaginatum, Barrande, Syst. Sil. de la Boheme, vol. ii. 
Texte V. p. 905. 
1882. Cyrtoceras (^Piloceras') invaginatum, Blake, British Foss. Ceph. 
pt. i. p. 186. 
1887. Piloceras invaginatum ?, Foord, Geol. Mag. dec. iii. vol. iv. p. 542. 
Sp. Char. Shell slightly curved. Section elliptical, judging by 
that of the siphuncle, the two diameters being in the ratio of about 
