380 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 4, 



I was formerly of opinion that some progressive changes could be 

 observed in the siphuncle of the Orthocerata ; but of this I have 

 not yet obtained entirely satisfactory evidence. 



In the work before referred to I stated that the Orthocerata did 

 not appear to have become decollated m their old age, and that "the 

 preservation of the shell was provided for by the increased size and 

 strength of the siphuncle, and its increased vascularity. In Endo- 

 ceras we find the siphuncle thickened by internal deposits, until (in 

 some of the very cylindrical species) it forms an almost solid axis.'' 



This last statement was founded on Prof. Hall's figures *, there 

 being no specimens of Endoceras in Europe. The diagram I gave 

 was ideal, and most likely incorrect ; for the internal tubes are pro- 

 bably invaginated siphonal joints (if anything) as suggested by Mr. 

 Salter. 



In the Chinese Orthoceras, now described, and in all the typical 

 species of the genus, the siphuncle is a simple tube, as in the recent 

 Nautilus, where it is nevertheless vascular and connected with a thin 

 membrane lining the air-chambers. But in those species which 

 have been separated under the generic name Actinoceras (including 

 Hormoceras and Huronia), the siphuncle possesses a complicated 

 internal structure, the appearance of which is liable to be modified 

 extremely by fossilization. In all these the structure is essentially 

 like that of the specimen figured and described by Mr. Stokes as 

 Hormoceras Bayfieldi (I. c. pl.60. f. 1), the vascular siphuncle being 

 divided into segments, which are radiately plaited and calcified. 

 The vessels which supplied the lining membrane of the air-chambers 

 (PL VI. fig. 3, s) passed through intervals or foramina between the 

 beads of the siphuncle ; in Actinoceras Bigshyi, and other Silurian 

 species, these foramina radiate equally from all sides of the siphuncle, 

 but in A. giganteuniy and others from the Carboniferous Limestone, 

 the foramina occur in only the ventral side of the beads. 



The reduplication of the vascular siphuncle is most remarkable in 

 " Orthoceras " trigonale (PI. VI. fig. 5) from the Devonian of Ge- 

 rolstein. 



The figure given by MM. d'Archiac and Verneuil f is taken from 

 a large specimen, and does not represent any peculiarity of structure ; 

 and those in the splendid work of the brothers Sandberger on the 

 Devonian fossils of the Rhine % are not so definite as the example 

 now figured, which was obtained by Sir R. Murchison, and given 

 by him to Mr. Stokes. This specimen seems to hav« escaped more 

 attention from having passed as a fragment of '^Cyrtoceras'' Eifeliense 

 (I. e. pi. 29. f. 1«), which has a similar siphuncle ; and probably 

 belongs to the same genus, although 0. triangulare has a straight 

 shell. 



With regard to the position of the siphuncle in the eccentric Or- 

 thocerata, it seems probable that such species would occupy, nor- 

 mally, an inclined position near the bottom of the sea, with the 



* Palaeontology of New York, vol. i. pi. 18. 



t Geol. Trans. 2nd series, vol. vi. pi. 27. f. 1. 



% Verst. des Rlieinischen Schichtensystems in Nassau, p. 155. pi. xvi. 



