ACTINOCERAS INSULARE. 31 



like swellings between the septa, and is traversed through the centre by the 

 endosiph uncle, whose radiating tubuli are given off circumferentially from these 

 swellings (PI. X, fig. 1 d). The siphuncle is somewhat compressed laterally, 

 perhaps through partial collapse, though it would be naturally less inflated than 

 in species having closer septa. Its outline, seen in section, is only slightly 

 inflated, so as to make each segment of it a little higher than wide. The necks of 

 the septa are distinctly seen in several places (PL IX, figs. 1 c, d ; PL X, fig. 1 d), 

 and in some places the perforations in the walls of the siphuncle (at the second 

 tubule from the bottom of the figure, PL X, fig. Id; the perforation is not 

 indicated in the drawing). The position of the siphuncle is markedly excentric, 

 being about two-fifths across the shorter diameter of the shell, measured to the 

 centre of the siphuncle ; it is also not quite central in relation to the longer 

 diameter. Its position may be best realised by looking at the figure (PL X, fig. 1 c). 

 The test is perfectly smooth. 



Size. — Length of the fragment 345 mm. ; greatest diameter 120 mm., least 

 45 mm. 



Affinities. — The fragmentary condition of the only example of this species 

 known to me up to the present time makes the question of its relationship with 

 other species a difficult one to settle. If the external characters are examined 

 the differences observed between the present species and Actinoceras giganteum 

 are found to consist in the smoothness of the test, the much greater width of the 

 septa, and the more compressed character of the segments composing the 

 siphuncle in the former as compared with the latter. In A. insulare the septa are 

 16 mm. apart where the diameter of the shell is 58 mm., whereas in a specimen 

 of A. giganteum from Orchard, near Glasgow, the septa, at the same diameter of 

 the shell, are only 13 mm. apart; or, measuring in another way, three chambers 

 of the Cork species require 43 mm. to span them, while in the Scotch specimen 

 only 34 mm. are necessary. The diameter of the shell for this measurement is 

 the same in both cases, viz. from about 50 mm. up to 58 mm. It would appear 

 also that the position of the siphuncle is more nearly central in A. giganteum 

 than it is in the present species. 



The obliquity of the septa, which is so strongly marked in A. insulare, is not 

 unknown in A. giganteum, and I find a reference in my note-book to a specimen 

 in the British Museum coming from Ireland in which this feature is well developed. 

 On the other hand, in the Belgian examples figured by de Koninck (' Faune Calc. 

 Garb.,' v, pi. xliv) the septa are perfectly horizontal. Reliance could not, there- 

 fore, be placed upon this character alone in distinguishing the two species, as it 

 seems to be one that is subject to variation. 



liemarks. — Attention should be directed still more particularly to the flatness 

 of the outline of the siphuncular segments in A. insulare as compared with 



6 



