174 PEOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



as in T, decussatus. The specimen comes from the Middle Eocene of 

 Brook in the New Forest. In contour and in sculpture it resembles 

 the Venus (i.e. Tapes) decussatus, figured and described by Deshayes in 

 1824,^ and stated to have been found at Orsay, south of Paris, in beds 

 now classed as Oligocene." 



The resemblance of T. Comptoni to the shell figured by Deshayes 

 and to the recent T. decussatus is curious, but, in view of the horizon 

 from which it was obtained, it is hardly likely to be really identical 

 with either. Moreover, Messrs. Cossmann and Lambert have expressed 

 their opinion that the shell figured by Deshayes is only a rather worn 

 specimen of the Venus Loeivyi, Meun., mentioned on p. 170.* It 

 should be remarked, however, that V. Loewyi has crenulated margins, 

 whereas T. decussatus has not. 



III. SUMMAKY AND CoNCLtTSIONS. 



It will doubtless be a long time before the workers in any branch of 

 zoology will agree as to what characters should be regarded as 

 essentially of generic value, and what are only of subgeneric or 

 sectional importance ; but such agreement can never be expected 

 unless everyone who studies a large family like the Yeneridae states 

 the reasons on which he bases his grouping. My study of recent and 

 fossil Veneridse has led me to form some definite opinions on the 

 relative value of the various special characters which have been used 

 as criteria, and these I now proceed to state. 



In the first place I am convinced that, in this family at any rate, 

 the characters of the hinge afford the best and most convenient means 

 of distinguishing the generic (and often the subgeneric) groups from 

 one another. By this I do not mean that other characters can be 

 neglected, or that a genus may be established solely because its teeth 

 exhibit small peculiarities of shape or position. I mean that the 

 hinge-plate, as a whole, exhibits a larger number of small but 

 appreciable differences than any other portion of the shell does ; the 

 development of the teeth from the primitive laminae of the plate 

 being a process which gives scope for a large number of variations or 

 combinations in the disposition of the broken-up laminae. 



It is really remarkable, too, how constant the teeth remain in size, 

 number, structure, and relative position in many large groups, not- 

 withstanding great variation in the shape of the shell. Take the 

 genus Callista, for instance, which includes some of the most trans- 

 versely elongate members of the family, as well as some in which the 

 proportion of length to height is only as 5 to 4, yet the teeth maintain 

 exactly the same relative positions, the only differences being in the 

 length of the posterior cardinals, and in the distance separating the 

 anterior cardinal from the anterior lateral. 



By some authors the differences exhibited by the pallial sinus are 

 still regarded as having much diagnostic importance, partly perhaps 

 because the pallial line is an expression of obvious differences in the 



' Op. cit., p. 142, pi. xxiii, figs. 8, 9. 



2 Terr. Olig. Marin d'Etampes : Mem. Soc. Geol. Fr., ser. m, vol. iii (1884), p. 80. 



