﻿116 BRITISH FOSSIL CEPHALOPODA. 



General Description.— Specimens of this species are usually compressed, but one 

 other besides the type shows a nearly circular section, in which case the rate of 

 increase is 1 in 10, though generally less as measured. The ornaments consist, first, 

 of the longitudinal convexities, which are usually, but not always, present ; in some 

 these run, as it were, in pairs, being separated by a slighter and a deeper depression 

 alternately, in others every fourth or fifth is stronger : secondly, festoons cross 

 these at regular distances ; these are sharp raised lamelke. The amount of curving 

 to form festoons is variable; sometimes they are mere undulations, sometimes the 

 concavities correspond both in size and position to the longitudinal convexities, and 

 sometimes they are independent of them. They vary also in distance from 1 line to 

 -1- line, independently of the size of the shell. Towards the aperture the shell is 

 thrown into irregular low folds, quite distinct from ornamental ribs, distant about 

 -jL the diameter (fig. 1) ; a thickening of the shell takes place here in one, and the 

 aperture is slightly oblique. Over the ornaments are preserved in some instances 

 longitudinal bands of colour (fig. 3), the darker bands corresponding to the convexi- 

 ties and the lighter ones to the re-entering angles, and they thus vary in breadth 

 with these. The septa are direct, or slightly undulating, and have a convexity of ^ the 

 diameter; they are distant ^ the diameter. The siphuncle is central, and in one 

 example has a diameter \ the whole, so that its elements should be as broad as they 

 are long ; but they have not been certainly seen in connection with the exterior shell. 

 There can, however, be little doubt that it is to this species that the siphuncles 

 commonly known as Orthoceras Brightii should be referred (fig. 2). It is true that 

 Phillips (loc. cit.) states that a large series in the collection of Mr. Lewis proves that 

 these siphuncles belong to 0. mocktreense, but that species has spherical bulbs. It 

 has long been suspected, and ultimately stated by Salter (loc. cit.), that this is the 

 siphuncle of 0. jhnbriatum. It could not belong to 0. annulatum, because the outline 

 of the shell in the matrix is invariably straight, nor is it likely to belong to a smooth 

 or slightly ornamented species, or its exterior would have been seen before now ; 

 but the upright lamellae of O.fimbriatum are just the suitable holdfasts to keep it 

 inextricably bound to the rock. These siphuncles agree in size and distance, and the 

 shell agrees in shape with O.fimbriatum. We thus learn that the siphuncle of this 

 shell has cylindrical elements a little less broad than long. They are affected as 

 little as possible by the septa, so that it is puzzling to know what could have 

 induced anyone to call them Actinoceras or Ormoceras. A section, however, shows 

 that there was a little obstructing deposit at the junction, which narrows the tube 

 internally, and whose outside is doubtless Sowerby's " plicated membranous bag." 

 This species attains a greater size than 0. annulatum, the longest being 20 inches, 

 and the greatest diameter under compression 3^ inches. The specimens are usually 

 large, but all the characters are observed without change down to a diameter of 

 | inch. 



