INTRODUCTION. 7 



ovate or ovately oblong, and the area is only slightly separated from tlie other portion of 

 the valve, thus resembling Unio. Mesially, or immediately anterior to the position of the 

 marginal carina, is a smooth space, which, commencing at the apex or near to it, gradually 

 widens downwards to the lower border; most commonly this smooth space is more 

 depressed than the other portions of the valve, it is also only very slightly impressed by 

 the lines of growth; its breadth in two of the species (T. Micheloti and T. Beesleyana) 

 is equal to all the remaining surface of the valve. The area is sufficiently defined and 

 has usually the bi-partite character of the preceding sections, but for the most part it is 

 destitute of carina? or has only indications of them near to the umbones. The anteal 

 portion of the valve has always costas more or less prominent ; usually they are much 

 smaller and more closely arranged or less strongly defined than in other sections ; they 

 are either plain or tuberculated, and not unfrequently both kinds of costae occur in the 

 same specimen. The lines of growth are very conspicuous near to the lower border, and 

 not uncommonly there are two or more arrests of development or transverse sulcations 

 separating the surface into as many zones, and influencing the direction of the rows of 

 costse longitudinally. The interiors of the valves have the dividing siphonal ridge 

 unusually prominent and forming a considerable indentation upon the internal moulds, 

 the ' Horse Heads ' of the Portland Oolite present good examples. Of the following 

 seven species the last only belongs to the Cretaceous Rocks: — T. gibbosa, Sow., T. 

 Damoniana, de Lor., T. Micheloti, de Lor., T. Manselli, Lye, T. tenui-texta, Lye, T. 

 Beesleyana, Lye, T. excentrica, Park. 



The preceding species, although they possess certain sectional features in common, 

 nevertheless appear to arrange themselves naturally into two groups, which may be 

 termed respectively the gibbosa group and the excentrica group. If we place those two 

 well-known species each at the head of a group, each of these series has in common the 

 plain wide mesial space which passes downwards from the apex to the lower border, but 

 in other important features the two groups appear to be sufficiently separated ; thus, the 

 gibbosa group, which is here illustrated by six species, has in the area and escutcheon a 

 repetition of the characters which are seen also in the Scap/ioidea, the Clavellata, the 

 Undulata, and the Quadrates ; both of these parts are well defined, and the area has the 

 usual bi-partite form, with bounding carina? more or less clearly developed. In the 

 excentrica group, on the contrary, there is no distinct escutcheon, and the space 

 representing the area is destitute both of ridges and depressions. Of this latter series we 

 possess only two British species, excluding T. sinuata, Park., and T. affinis, Sow., which 

 are only synonyms of T. excentrica, Park. T. longa, Ag., must also be referred to the same 

 group. It is, therefore, not without some hesitation that I refrain from proposing to add 

 another to the already numerous sectional forms of this genus, embracing T. excentrica 

 and T. Beesleyana, and content myself with looking forward to the probability that the 

 acquisition of additional forms will at some future period induce changes in the sectional 

 arrangement in the direction now indicated. 



