﻿298 DR. A. VATTGHAN ON THE PAL^SONTOLOGICAL [May 1905, 



A CUothyris has the internal characters common to all Athyrids, 1 

 namely : — 



In the pedicle-valve : strong dental plates which form the tall, vertical walls 

 of the pedicle-cavity. Elongated dicluctor-impressions, ruled lengthwise by 

 strong parallel ridges (see fig. 5). 



In the brachial valve : a strong mesial septum, with two pairs of narrow 

 adductor scars on either side. 



Reticularia aff. lineata (see below, p. 299), which has, approxi- 

 mately, the same range as CUothyris glahristria, has the same 

 general form and the same internal characters (ennmerated above); 

 consequently, specimens of CI. glabristria which retain the fringes 

 have a remarkable resemblance to Reticularia aff. lineata. When 

 the beak-region can be clearly made ont, it is easy to distinguish 

 the two species, by the fact that the apex of the beak in a Reticularia 

 is typically non-perforate and the hinge-area is well developed. The 

 two species are also satisfactorily separated by the under layer of 

 fine close-set, radiating, scabriculate ribs which characterize the 

 genus Reticularia ; but even this character is somewhat simulated 

 in the Athyrid by the occurrence of distant, radiating ridges beneath 

 the outer layer of the test, which are represented on the cast by 

 faint ridges. 



Cliothteis glabristria, mut. 



At the top of Z 2 we meet with elongate forms, transitional towards 

 Seminula ficoidea, in which the characteristic fringes are merely 

 suggested by faint striae and the shell has acquired the tall beak 

 and sloping shoulders of S. ficoidea. 



In S., of Burrington Combe, Weston, 2 and Lydstep an Athyrid 

 occurs which exactly simulates CUothyris glabristria in general 

 form. The outer layer of the test is, however, produced into 

 imbricating expansions, which do not form fringes as in a CUothyris. 

 Each of the expansions is a continuous sheet, with broad, low radial 

 ribs ; and specimens from which the expansions have been removed 

 still exhibit a characteristic broad reticulation. 



This form must be named Athyris cf. glabristria ; it bears the 

 same relationship to Athyris planosulcata that CUothyris glabristria 

 does to CUothyris Royssii. 



I have accepted with some hesitation Davidson's dictum that the 

 figure to which reference is made above actually had a spinose 

 investment, seeing that most of the specimens that I have examined 

 from Midland and Northern localities appear to me to belong to 

 Athyris, and not to CUothyris. 



Seminula. 



The internal characters are the same, as regards septa and scars, 

 for all Athyrids, and are illustrated by the diagram of CUothyris 



1 These characters are less developed in CUothyris Royssii than in CI. glabri- 

 stria, so that it is not difficult to separate the transverse variant of CI. Royssii 

 that rarely occurs in K x , from a typical CUothyris glabristria the external 

 form of which it simulates. 



2 Mr. T. F. Sibly discovered this form at Weston, where it is abundant in 

 certain beds. He has kindly allowed me to examine and describe his specimens. 



