BRITISH FOSSILS. 17 



" the external shell not exhibiting the radiating fracture of the latter, 

 " and I have given it a specific name from its great length in proportion 

 " to the diameter. Mr. White, to whom I am indebted for the specimen, 

 " informs me that it was originally considerably longer than at present." 



As this specimen (now in the Museum of the Geological Society) is 

 by no means well represented in the plate accompanying De la Beche's 

 memoir, I have had a correct sketch of it made (Plate IH., fig. 3). 



It consists of an imperfect sub-cylindrical guard 3*2 inches long; 

 fractured above and below, and having, in its lower part, a diameter 

 of rather less than one-fifth of an inch. It contains the remains of a long 

 tapering phragmacone, the chambers of which have been completely 

 filled with transparent calcareous spar. The rounded, bead-like apical 

 chamber of the phragmacone, not one-fortieth of an inch in diameter, is 

 situated at about 0*2 of an inch from the fractured extremity of the 

 guard. The chambers gradually increase in length and in breadth, until 

 at 2 25 inches from the apex they are 2 of an inch long by 25 wide. 

 Beyond this point, the phragmocone is broken away, but the impressions 

 of three chambers are left on the inner surface of the eonotheca, which 

 adheres to what remains of the attenuated, upward, prolongation of the 

 guard. Altogether, there seem to have been about 30 chambers in 

 the 2*9 inches length of phragmocone, but the 10 outermost chambers 

 take up 1 * 8 inches of this extent. The conotheca is a thin lamella of 

 a much paler colour than the guard, to the walls of the alveolus of 

 which it adheres. 



The real nature of this " Orthoceras " was first revealed by the 

 beautiful specimen obtained by Mr. Day, which is represented in 

 Plate HE., fig. 1, reduced to four-fifths of the natural size. Here, the 

 apex of a phragmocone of similar character is inclosed within a 

 subcylindrical guard, obtusely truncated at its free end. For an inch 

 and a half from its distal end, this guard is entire, but beyond this 

 point (6) it is split, and the dorsal has come away from the ventral 

 half, leaving the phragmocone (c) exposed. The chambers of the 

 phragmocone are filled with transparent spar, and their casts, thus pro- 

 duced, are exposed to view inferiorly. Superiorly, they are coated 

 over by a thin pellicle of quite a similar character to the theca of 

 the phragmocone in the original specimen ; and, indeed, at the sides, 

 this layer dips down between the fractured edge of the guard and the 

 phragmocone, showing clearly that it is the conotheca. 



At 3-6 in. from the end, the specimen is transversely fractured, and 

 the section of the guard cannot be traced further than the fracture; but 

 a layer of shelly matter (a) quite similar to that which forms the cono- 

 theca, and which was obviously continuous with it, coats what appeal's 

 to be the upper termination of the phragmocone, and passes into the 

 remarkable pro-ostracum, the extreme point of which is broken off : 

 when it was entire it measured about 11^ inches. Posteriorly it is 

 a flat band 0.35 in. wide, which slowly narrows until its width is 

 about 2 in. ; it then widens to 5 in., and, finally, gradually tapers to 

 its point. Where it widens it thickens, and its surface, from being flat, 

 becomes convex from side to side, so that its section acquires the 

 form of a not very depressed ellipse, and this form is retained close to 

 the apex at/. 



The surface of this singular pro-ostracum is polished, but is covered 

 with transverse wrinkles, or ridges, which are especially numerous 

 where the flat portion passes into the rounded part. 



The sections of the pro-ostracum exhibit its structure. Like the guard 

 of an ordinary Beleinnite, it is composed of concentric lamella?, each 

 of which consists of fibres disposed perpendicularly to the plane of the 



