32 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



A. giganteum, in which they are strongly inflated or bulbous (cf. PI. IX, fig. 2 c, 

 with PL X, fig. 1 d). The greater width between the septa in A. insulare may 

 have caused the segments to be more drawn out, and thus to approach the 

 cylindrical form which they must have assumed if this process had been carried 

 still farther; whether, however, this would have been compatible with the 

 existence of the endosiphuncle and its appendages is questionable, since it is 

 evident that the development of these organs could not have taken place within 

 a very contracted space. It is at least certain that in such a form as A. insulare 

 there could not have been developed so great a number of tubules as are indicated 

 by the perforations in such a form as Actinoceras Bigsbyi (cf. 'Cat. Foss. Ceph. 

 British Museum,' 1888, vol. i, p. 164, fig. 21). 



I may take the opportunity before leaving the subject of the structure of 

 Actinoceras to refer to an important contribution to the literature of the fossil 

 Cephalopoda by Prof. Hyatt, viz. his " Phylogeny of an Acquired Characteristic " 

 (' Proc. Amer. Phil. Soc.,' vol. xxxii, No. 143, August, 1894). Under the heading 

 " Ontogenetic Stages," in which the embryology of the group is very fully 

 discussed, some important observations are made with reference to the siphuncle 

 of Endoceras, Piloceras, and Actinoceras, and justification is found for the use 

 of the term endosiphon (or endosiphuncle), to which F. A. Bather, in his able 

 critical summary of recent views and discoveries (" Cephalopod Beginnings," 

 ' Natural Science,' vol. v, December, 1894), takes exception. The following extract 

 from Hyatt's memoir has a direct bearing upon the subject : — " The structure of 

 the apex in Endoceras, Piloceras, and Actinoceras indicates large and direct, open, 

 tubular connection between the protoconch and the animal when in this first 

 chamber through which the endosiphuncle in the generalised Nautiloids, Endo- 

 siphonoidea, opened into the protoconch. The tubular opening of the apex in 

 Endoceras, Piloceras, and Actinoceras, and other genera having a marked endo- 

 siphuncle, is not closed by the caecum of the siphuncle as was formerly supposed. 

 It is, on the contrary, directly continuous with the endosiphuncle, as was first pointed 

 out by Foord in his 'Catalogue of Fossil Cephalopoda in the British Museum,' 

 part 1, 1888, p. 165. This is an attenuated, central, more or less irregular tube 

 or axis formed by the extension of the points of successive endocones or sheaths. 

 It is more or less interrupted by pseudo-septa, and is a separate and distinct part 

 occupying the axis of the large siphuncle. This organ is continuous with some 

 corresponding part in the embryo which existed in the protoconch. On the other 

 hand, the true siphuncle, including the caecum of the first air-chamber, is a 

 secondary organ formed by the funnels of the septa." 



The " endocones or sheaths " and " pseudo-septa " referred to by Hyatt in the 

 above quotation do not occur in Actuioceras or its congeners, but the analogy 

 between the inner tube in the siphuncle of the latter and that which is found in 



