﻿BRITISH FOSSIL CEPHALOPODA. 203 



is considerable, the mean radius being 2 inches when the mean diameter is 1^ inches. 

 The rate of increase on the septate portion is a little more than 2 in 5. The length 

 of the body-chamber is uncertain, the septa mostly being covered. The curvature is 

 continued without change to the aperture on both sides. The aperture consists of a 

 large opening which is transverse, the diameters being as 3 to 2, — this projects both 

 forwards and outwards ; of a long passage which is quite closed and makes the upper 

 surface nearly or quite flat ; and of a small aperture of elliptic shape, with the long 

 axis ventro-dorsal, more or less produced ventrically. The shell has upward 

 imbricating lamellae, running sigmoidally across the side, so as to come to a broad 

 curve on the convex border, and on the concave border bending rapidly and 

 suddenly back into a quadrate backward pointing tongue, which lies in a well- 

 marked groove. These imbrications are but slightly elevated, and are distant 

 about ^ of a line. No crenulations have been seen at the base of the body-chamber. 

 The septal and siphuncular characters are not very certain. The greatest length is 

 4 inches, and greatest diameter 2^ inches. 



Relations. — The greater curvature, more rapid increase, peculiar ornaments, and 

 differences of septal character separate this species very clearly from P. ventricosum. 

 It is much more nearly allied to P. arcuatum. Indeed, as the present species is 

 nearly peculiar to the Wenlock Limestone, and the last named characterises the 

 Ludlow Series, it seems probable that the one is the direct descendant of the other, 

 having lost some of the peculiar characters. These are the regularity and fineness 

 of the upward imbrications — their coming to a tongue-like curve on the concave 

 side situated in a depression, the greater curvature and more rapid increase, and the 

 outward direction of both apertures. In the other characters they show a singular 

 agreement. The crenulations at the base of the body-chamber, which are so constant 

 in P. arcuatum, have not been seen in British specimens of this species, though 

 Barrande notices them in some from Bohemia. 



Distribution. — In the Wenlock Limestone of Ledbury (8) and of Dudley (5), 

 and possibly a fragment in the Lower Ludlow of G-arcoed, Usk (1). 



Phragmoceras obliquttm, Blake, PL XXIY. fig. 7. 



Type. — The section is oval, the larger end being toward the convex side, and the 

 ratio of the axes being about 2 to 1. The rate of increase on the septal portion is 

 not very great — about 1 in 3. The curvature has a mean radius of about 1^- inches, 

 which is greater than the diameter of the shell. The body-chamber continues the 

 curve of the sides, but its length is less than its basal diameter. The line of 

 aperture is oblique, about 40° with the base of the body-chamber. Both apertures 

 are rather large and almost transverse, but are broken down. They are joined by 

 a perfectly closed passage. No ornaments are seen. The greatest diameter is 



2 d 2 



