﻿lOS 



BRITISH FOSSIL TRIGONIvE. 



higher than tliose which are characterised by the typical form ; the former are also found 

 in a better condition of preservation ; their profuse and varied ornamentation command 

 general admiration, and require several specimens to exemplify adequately their several 

 aspects ; this variability, although considerable, scarcely affects the general figure, and is 

 nearly limited to the ornamentation of the surface ; but vi^ith so much irregularity in the 

 nodes, description must to some extent become subordinate to figures. The diff'erence of 

 figure supplies the most constant and clear distinction which characterises the two 

 varieties. In the typical form (Plate XXV) the area constitutes a moiety of the valve, 

 and the pallial or costated portion has somewhat less breadth than in the other ; the 

 great length of the siphonal border imparts consequently a more quadrate figure to the 

 shell. The variety Orbigityana has the costated or pallial portion larger than the area ; 

 the siphonal border passes more obliquely downwards, or forms a more obtuse angle with 

 the upper border ; the lower or posteal extremity of the valve is therefore more produced 

 and pointed. 



The internal moulds are only known to me from the small specimens of T. cincta 

 figured by Agassiz ; in these the height is less in proportion than in specimens not 

 deprived of the test. 



Dimensions. — Specimen of the typical form from the Perna bed at Red Cliff, 

 Sandown. 



Length measured upon the marginal carina 



At right angles to the carina across the pallial surface 



Across the area .... 



Length of the siphonal border . 



Length of the escutcheon 



Dimensions of a specimen from Hythe : 

 Length upon the marginal carina 

 At right angles to the carina across the pallial surface 

 Across the area .... 

 Length of the siphonal border 

 Length of the escutcheon 



Variety Orhignyana .■ 



Length upon the marginal carina 



At right angles across the pallial surface 



Across the area 



Length of the siphonal border 



Length of the escutcheon 



Diameter through the valves . 



Breadth of escutcheon 



