﻿BEITISH FOSSIL CEPHALOPODA. 107 



inner construction is altogether like that of 0. imbricatum " [i.e. the siphuncle is 

 small and central], " but it has about twenty longitudinal ribs, and is scarcely concave 

 between them." 



General Description. — The shells of this species are so constantly found flattened, 

 that the true shape of the section is difficult to determine ; in none has it been seen 

 circular (see PL VII. fig. 8) ; but in the least compressed, which is Sowerby's type 

 specimen of 0. canaliculatum, the ratio of the diameters is 22 to 25, the shorter one 

 being in the plane of symmetry. In some examples there is a slight show of curva- 

 ture, but this may be due to contortion ; notably in both the types of Sowerby 

 curvature is seen, though in 0. canaliculatum there is less than in the other. The 

 mean rate of increase is 1 in 9 on the septate portion, decreasing to zero towards 

 the aperture. The body-chamber is a little longer than the diameter of its base, and 

 attains a length of 2 inches ; it is nearly cylindrical, but below the aperture there 

 is a broad, shallow, straight depression, and then a slight expansion. The aperture 

 is nearly direct and straight, but there is a slight convexity in its outline on the 

 ventral side (PI. VII. fig. 1). The ornaments show two rather distinct varieties. 

 In the ordinary forms to which the English names were assigned, there are from 

 32 to 42 acute longitudinal ribs per circumference, whose intervening spaces are 

 uniformly concave, so as to give the surface a fluted appearance ; in the variety 

 to which Wahlenberg's type would seem to have belonged, the ribs are fewer in 

 number, from 20 to 24, and the intervening spaces are almost flat. This is of much 

 rarer occurrence than the other. The space intervening between the ribs is marked 

 by fine sub-imbricating transverse riblets, from 10 to 48 per line, according to the size 

 of the shell, about 36 being in a space equal to the interval of the ribs : these have a 

 convexity towards the aperture. The septa are a little oblique, but when seen on 

 the dorsal or ventral side look almost direct. Their convexity is slight, not exceeding 

 A- the diameter when the shell is not flattened. They are separated by rather 

 irregular intervals, varying from f- to \ the diameter, becoming closer in proportion 

 with age, and the last chamber is diminished to half the usual size. The siphuncle 

 is always very near the centre ; and though it passes in some cases slightly out of 

 the central line, it never becomes in any sense lateral. Its diameter on the surface 

 of the septum is y 1 ^- to y 1 ^- the diameter, but it expands between the septa into small 

 cylindrical bulbs, whose breadth is equal to their length, and their surface smooth. 

 The greatest length of the shell actually seen is 5 inches ; but since, in one example 

 showing this length, the greatest diameter was only ^ inch, and others have a 

 diameter of If inches, the total length of such an one would be 15 inches or more, 

 since the growth diminishes with age. No really good examples of this species 

 have anywhere been seen, nor has the apex been preserved in any in such a manner 

 that its characters might be learned. There is a very small specimen in the Museum 

 of the Royal College of Science, Dublin, which may represent the young of this or 



p 2 



