﻿BRITISH FOSSIL CEPHALOPODA. 205 



seems to be constant up to the aperture, with a very slight expansion near the 

 convex side. Hence this has been taken for a Cyrtoceras, but it appears to contract 

 in the transverse diameter. The rate of increase is 1 in 3. The body-chamber has 

 a length equal to its basal diameter. The aperture is constricted in the middle, and 

 Sowerby says the " beak " is " direct " ? The ornaments are sharply marked lines 

 of growth. On the base of the body-chamber is a crenulated band. The septa 

 have a very moderate convexity, and are distant y the mean diameter, the last being 

 of half size. The sutures are nearly straight, only slightly concave towards the 

 aperture. The siphuncle is internal and its elements are bulbous, their diameter 

 being ^ of that of the septum. The length is 2§ inches, and the greatest diameter 

 is If inches. In the Lower Ludlow of Ludlow, or Ledbury. 



General Description. — It is to a great extent by inference that this name is 

 adopted, the original specimen being undiscoverable. It is unlikely so well-marked 

 a shell should be unique, especially when it is said to be found at Ledbury, a locality 

 so well worked by Dr. Grindrod ; and there is a species occurring there agreeing 

 with Sowerby's description and figures, and separable from P. ventricosum. For this 

 (now better known) species we may adopt the name of P. arcuatum. 



The section when best preserved is oval (PL XXVI. fig. Id), with the longer 

 axis in the plane of curvature, the wider side on the convexity of the shell, the 

 greatest thickness being at f- of the diameter towards the outside. The ratio of the 

 diameters is 11 to 8, but when flattened they are of course less equal. The mean 

 radius of curvature is 2| inches when the mean diameter is 2^ inches. The rate of 

 increase remains the same throughout in the ventro-dorsal plane, but transversely 

 the body-chamber draws in to the aperture ; the rate is 1 in 3^ measured along the 

 convex curve. The body-chamber has a less length than its basal diameter, but not 

 far removed from it. The aperture consists of a large opening which is transversely 

 uniform, and is situated on a prolongation of the shell, which closes in more 

 ventrally, but continues its normal curvature here ; of the passage which is closed 

 pretty accurately for nearly 3 times the ventro-dorsal length of the large opening, 

 and is parallel to the base of the body-chamber ; and of the small opening which is 

 produced horizontally and so becomes sub triangular. The ornaments of the shell 

 are rough risings of growth which are easily lost, are not very regular, but which 

 curve back very much towards the convex side, cutting the septa at an angle of 60°. 

 On the base of the body-chamber in all specimens seen is a crenulated band, which is 

 more or less feebly continued upon the lower part of the body-chamber. As a rule, 

 I am not inclined to consider these marks as a specific character, for they may be 

 absent or present ; but in this species their presence seems always accompanied by 

 the other distinctive characters, and I have not yet observed them in specimens 

 certainly belonging to P. ventricosum. The septa are direct, with very little 

 concavity of suture ; their convexity is \ the long diameter, and their distance \, or, 



