﻿CLAVELLAT^.. 



43 



Area narrow, distinctly bipartite with three dehcate tuberculated carinas and irregidar 

 transverse plications ; there are also some irregular varices near to the posteal portions of 

 the median and inner carinse ; those of the latter extend a little upon the escutcheon, 

 The ornamentation upon the other portion of the valve varies much in accordance with the 

 development in the growth of the shell. A specimen two and a half inches in length has 

 twelve rows of tuberculated varices, which rise nearly perpendicularly to the carina ; anteally 

 they curve much forwards, and are continued in an attenuated condition almost to the 

 anterior border ; the superior portions of the varices have the tubercles large, rounded, 

 but somewhat unequal and irregular ; anteally they rapidly become small and cord-like ; 

 the lines of growth are strongly marked over the whole of the shell. A specimen three 

 and a half inches in length figured by Messieurs de Loriol and Pellat has the costge more 

 broken near to the anterior border; they lose all distinctness and form an irregular 

 assemblage of small tubercles. The original figure given by Miss Benett represents a 

 condition of growth still more advanced, the anteal gibbosity has much increased, and its 

 border is almost destitute of tubercles ; the plications of growth have also become more 

 conspicuous. A specimen greatly larger and still more inflated from Niangle, Boulogne, 

 is No. 36913 in the British Museum; the anteal folds of growth have here replaced all 

 ornamentation. 



In the Portland Oolite at Swindon and at the Isle of Portland internal moulds are 

 common, but it rarely happens that any considerable portion of the test is adherent ; the 

 characters of the surface may, however, be ascertained upon the adherent portions. The 

 examples figured 5 and 6, Plate IX, represent the more frequent condition of such 

 specimens. Figs. 4 and 5, upon the same plate, may be regarded as a distinct variety 

 from the Kimmeridge Clay, with the anteal 

 portions of the varices curved and unbroken. 

 No. 2 has suffered somewhat from vertical 

 pressure, its umbo is more than usually 

 produced and pointed. 



In the Portland Oolite of Boulogne it 

 has occurred in a better condition of pre- 

 servation and of much larger dimensions; 

 the original of our wood engraving is from 

 the latter locality, and represents an example 

 of adult growth, but not of the largest dimensions, the size is reduced one half; the small 

 anteal supplementary varices are unusually nmnerous and strongly defined. 



