﻿BRITISH FOSSIL CEPHALOPODA. 91 



character of age ; for some of the largest specimens have the strongest ribs, and 

 the sudden change seen in the type is not repeated in any known British example. 

 The transverse festooned riblets are, when well preserved, upright lamellse; the 

 portions which are concave to the aperture are much wider than the other portions ; 

 *n distance, and relation to the longitudinal elevations, they are very variable, gene- 

 rally they are most marked when fewest in number. They are more crowded on the 

 ribs than in the interspaces, and the more usual numbers are from 7 to 9, the lowest 

 being 6 and the highest 18, for each rib and interval; the breadth of the festoons 

 is from |to | the rib interval, more frequently the latter. There are occasionally 

 fine parallel strise between these riblets. Sometimes rounded, longitudinal, low ele- 

 vations are developed, especially between the ribs ; the relations of the festoons to 

 these are very variable : sometimes the part which is convex to the aperture lies on 

 the elevation, sometimes between : sometimes the regular succession of the festoons 

 gives a false appearance of longitudinal lines, and occasionally there is no sign of 

 them. In addition to these ornaments, one example shows bands of colour (fig. 4). 

 It is an example in which the fimbriating riblets are numerous, 15 per space, very 

 feebly festooned and showing no signs of any longitudinal elevation. On one side 

 of it are bands ■§- inch broad, alternately light and dark, running in a longitudinal 

 direction. The ornaments of the exterior do not appear to invariably mark the in- 

 ternal cast, but sometimes to leave it nearly unribbed. The septa cut the surface on 

 the upper slope of the ribs, and thus are dependent on them for their distance, &c. ; 

 the convexity of the septal surface in the type is a maximum, it is usually less, and 

 the septa are of the ordinary shape. One specimen (PL VIII. fig. 4), of two septal 

 chambers without external surface, which would seem to belong to this species 

 rather than to any other, shows a curiously conical form of septal surface. The 

 siphuncle, when seen, is usually central, but it occasionally approaches one side, 

 as in fig. 8. It does not increase in size at the same rate as the shell, so that it 

 has a smaller proportion to the diameter in larger examples, the average being 

 from ^ to -^ ; it expands but slightly between the septa to about 5 the diameter, 

 and is cylindrical in shape. The longest seen is 13 inches, and the greatest diameter 

 1\ inches. 



Relations. — The peculiar features of this species are so well marked, that the only 

 question is whether its variability is sufficient to admit within its limits the forms 

 called O.Jimbriatum ; the negative of this will be maintained under the heading of 

 the latter. With regard to 0. attenuatum, for which, if it be a true species, a new 

 name would have to be invented, as its present one is pre-occupied, though the type is 

 said to be in the Ludlow Museum, it is no longer to be found there, and I have been 

 unable to discover it. As no description is given of it by Sowerby, and the drawing 

 is at second-hand, and no specimens have been found elsewhere that would match 

 better with that figure than with any other, I conclude it belongs to some known 



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