﻿GLABRAE. 



95 



costse everywhere evanescent near to the pallial border, where the surface is occupied 

 almost solely by the lines of growth, which are large, irregular, and rather distantly 

 arranged. Several sulcations or arrests of growth are usually visible at irregular 

 intervals. The costse have their anteal portions horizontal or directed slightly down- 

 wards ; this excentrical direction has been depicted both by Parkinson and by Sowerby 

 in a manner somewhat exaggerated where the costse are crossed by oblique lines of 

 growth. 



The length compared with the height is as ten to seven, or, as in other examples, as 

 ten to eight. 



The hinge-teeth diverge widely, and are larger than is usual in the Glabra; the 

 adductor scars are deeply impressed, more especially the anteal adductor, which forms a 

 deep sinus, passing upwards towards the apex of the valve concealed by the anterior 

 dental process in each valve ; the borders of the valves are plain, their inner surfaces 

 exhibit remains of the iridescent nacre in well-preserved specimens. 



The figures upon Plate XX illustrate the young condition of the species; fig. 6 

 closely resembles the T. sinuata of Parkinson ; fig. 5, a specimen of much more advanced 

 growth, retains the surface ornaments similar to the smaller specimen; Plate XXI, fig. 6, 

 and Plate XXII, figs. 5, 5 a, represent common examples of T. excentrica in which the 

 umbonal portion of the test does not retain the characters of the surface ; the large 

 specimen, Plate XXI, fig. 7, which agrees with the T. affinis of the ' Mineral Conchology,' 

 has the valve thickened from advanced growth, and the horizontal costae are obscure. 

 Much of the variability seen in this species is produced by differences in the general 

 figure which are not dependent upon any one stage of growth ■ thus, the short and thick 

 example fully developed, depicted upon Plate XXI, which fairly represents the T. affinis 

 of the ' Mineral Conchology,' is nearly allied in figure to certain young forms which are 

 only three or four lines in length ; these latter also pass gradually into the undoubted 

 young condition of T. excentrica, Plate XX, fig. 6, which is more lengthened. The six 

 figures upon our plates, although exhibiting much diversity of aspect, do not sufficiently 

 exemplify the medium-sized and fully developed forms of the more lengthened specimens ; 

 through the kindness and discrimination of Mr. Vicary this defect may be rectified, that 

 gentleman having recently forwarded to me so considerable a series of specimens from the 

 Greensand of the Blackdown region as to enable me both to verify the unity of these 

 three supposed species and to select from them specimens exhibiting the surface 

 ornaments of the umbonal portions of T. excentrica and their identity with the small 

 specimens commonly referred to T. sinuata. These will be given upon a future 

 plate. 



Dimensions. — Length of the largest of our specimens 2f inches, height 2\ inches, 

 convexity of a single valve f inch. Occasionally the species attains larger di- 

 mensions. 



Affinities — The umbonal portion of the shell in the posteal sinuation of its delicate 



