﻿114 BRITISH FOSSIL CEPHALOPODA. 



where most of the figured specimens of the Silurian system were deposited. We 

 have, therefore, only the figure and description in that work. That description is, 

 " Rather quickly tapering, longitudinally ribbed ; ribs fine, numerous ; septa 

 numerous. This differs from the 0. striatum (0. bullatum) in the coarseness of the 

 lines upon the surface, which in this are sharp elevations. The position of the 

 siphuncle is unknown. Length, 1^ feet ; greatest width, about 4 in. Ludlow 

 escarpments." The figure shows also that the section is probably circular; the 

 tapering at a diameter of 3^ inches is 1 in 9. There are about 60 to 64 riblets 

 in the semi-circumference ; the septa are direct, ^ the diameter apart at first, 

 and later only -j^ of it. 



General Description. — The specimens of this species are usually of large 

 dimensions, and in this case their characters are well marked, but the true nature of 

 the young is rather obscure. The section is circular, though generally compressed. 

 The rate of increase is at first 1 in 5, but decreases through the average of 1 in 9, to 

 be at last only 1 in 12 in the body -chamber. The latter reaches a length of 12 

 inches, which is four times the diameter of its base. There is a shallow constriction 

 surrounding the aperture, which is direct and nearly straight. The ornaments are 

 very distinct, being narrow riblets set on a flat surface (fig. 8a), with usually no 

 intermediate transverse lines. These in large examples are pretty constant at 120 

 in the whole circumference. They are occasionally slightly irregular, and here and 

 there a finer intermediate riblet may be seen. The septa are direct, and have only 

 a moderate convexity ; they are distant f- the diameter apart at the smaller end, but 

 gradually become closer, the last few being crowded into ^ the diameter each. The 

 siphuncle is central (fig. 9) ; its elements are butt-shaped. The largest specimen 

 critically examined is the one figured, which is 15 inches by 3^ inches ; but there 

 are examples in the Ludlow Museum and elsewhere over 2 feet in length. 



Var. spectandum, PI. VII. fig. 12. — A remarkable fragment, which may at 

 present, at least, be placed under this species, shows very similar ornaments to those 

 of 0. spectandum (Barrande, pi. 266), and different from the type. The primary 

 ribs are irregular and rather more numerous, and each is furrowed along the top ; 

 between each pair of these lies a very fine parallel riblet, and the deeper spaces are 

 transversely striated. This style of ornament when magnified resembles that in 

 0. Bacchus, but on this large fragment the primaries are too small for that species. 

 The siphuncle of this specimen is f- across the diameter ; but this may be due to 

 pressure. It is from the Wenlock Limestone. 



Relations. — This bears very much the same relation to 0. lineatum of the Bala 

 Beds as 0. Bacchus bears to 0. coralliforme. It attains a much larger size, and in 

 doing so puts on features which justify us in considering it a distinct species. When 

 the primary riblets of 0. Bacchus lose their importance, it approaches very closely 

 to this species, which, however, retains the same ornaments from an early stage. 



