﻿112 BRITISH FOSSIL CEPHALOPODA. 



ribs pass over them ; 4th, very fine transverse striations, found only in the interval 

 between the riblets. The septa have a convexity of ^ the diameter ; they are 

 horizontal, and at an average distance of | the diameter. The siphuncle varies in 

 position in the same individual. It begins by being halfway between the centre and 

 circumference, and ends by being central, but the variation is not uniform. It is 

 slightly inflated between the septa to about ^ the diameter ; the elements lie a little 

 obliquely. The largest specimen is about 21 inches long, and 2 inches in diameter 

 at the base of the body-chamber. From the Upper Silurian, B 2, of Bohemia. 



General Description. — This magnificent species is abundantly represented in our 

 British Silurian rocks. Almost all specimens are compressed, and the true shape is 

 therefore doubtful. In the least compressed body-chamber that I have seen, the 

 section is rather quadrate, and has diameters in the ratio of 11 to 15 ; the actual 

 ratio seen in most examples is not more than 1 to 2. In examples of less diameter 

 than 7^ lines, there is a perceptible curvature in the plane of the minor axis, and 

 one even of large size shows a similar feature. The body-chamber has a length 

 of twice its long and four times its short diameter in compressed examples. It shows 

 a diminution in the rate of increase, or even a lessening diameter towards the 

 aperture, which is surrounded, in one, by a feeble constriction. The ornaments 

 consist, first, of primary longitudinal riblets, varying in number from 24 to 40 per 

 whorl according to size, and partly according to individuals ; secondly, there are 

 smaller riblets of similar character which rise between the first, and gradually 

 assume the same dimensions ; and thirdly, one to three still smaller riblets lie in the 

 concavities : all these are well separate, and, though acute when young, constantly 

 show a groove along the top towards the large end, as though they had been hollow 

 and worn. Another peculiarity is that the tertiary riblets may be traced growing 

 gradually equal to the secondaries, and both to the primaries, so that on the body- 

 chamber the distinction is almost lost (fig. 3), and in the young it is not completely 

 developed (fig. 5). In some instructive examples of young forms of this species 

 from the Pentland Hills, transverse undulations rising almost to ribs divide the 

 surface, with the primaries, into squares, but these ribs fade away at a diameter of 

 5 lines. The same is seen in a specimen from Dudley (fig. 7) Besides these, there 

 are very fine transverse strife from 9 to 12 per line, that occasionally, but not 

 constantly, cross the interspaces between the riblets. An example from the Upper 

 Silurian of Kerry, at present referred to this species, has the secondary riblets so 

 even in size, and the transverse ones so strongly marked, as to approach closer in its 

 ornaments to 0. Stokesi, Barrande. The whole of these ornaments show a tendency 

 to die away near the aperture, and to be succeeded by transverse lines of growth 

 only. The septa have but a moderate convexity, about ^ the diameter, but increasing 

 by compression to twice as much. They are very nearly direct, no greater obliquity 

 than 10° being produced by compression. Their distance varies much with age, being 



