JL'KKS-BKOWNE : OX TIVELA AND GRATELOUPIA. 271 



not only mention tlie existence of this supposed posterior lateral as 

 a generic cliaracter, but show its position in their diagram of the 

 hinge of G. irregularis. In their description of this species, however, 

 they write of it as " tres rudiinentaire, la fossette opposee etant tres 

 indecisee", while under G. clifficilis they remark tliat there is "pas 

 d'apparence de lamelles posterieures, le bord superieur est seulement 

 un peu rainure". 



Til us these autliors admit tliat the supposed posterior lateral does 

 not exist in G. difficilis, and that it is very rudimentary in G. irregularis ; 

 hence it is impossible to regard it as a generic feature. Having 

 examined good specimens of both species, which I owe to the kindness 

 of Professor Peyrot, I can (luito confirm their statements; I am not 

 surprised that they regard the tooth as vei'y rudimentary, even in 

 G. irregularis (Fig. 5), for I feel sure that it has no real existence. 

 The feature which has given rise to the idea of such a tooth is a slight 

 inflection of tiic posterior margin of the valve beyond the end of the 

 ligament, and a thickening of the inner border of the groove which 

 exists in all species of Tivela iind Meretrix. The extent of this 

 thickening varies with the individual and with the age of the shell, 

 and it has been exaggerated in some figures of the shell, notably in 

 that given by Hoernes (op. cit., pi. xvi, fig. 56). Moreover, no 

 one has claimed to recognize a corresponding tooth on the posterior 

 border of the left valve ; no elevation exists on that border, and its 

 absence militates strongly against the existence of a lateral tooth in 

 the right valve. 



As, however, I did not wisli to dissent from such authorities as 

 Messrs. Fischer, Cossmann, and Peyrot without confirmation of my 

 opinion, I sought that of Messrs. E. A. Smith and 11. B. Jfewton, of 

 the British Museum, Avho have kindly examined the specimens of 

 G. irregularis in the National Collection, and have informed me that 

 tliey agree in considering tliat " there is no posterior lateral tooth 

 in Gratdoupia^\ 



This being so, there is really no essential generic difference between 

 Grateloupia and Tivela ; indeed, the differences are scarcely of sub- 

 generic im])ortance. The dentition of Grateloupia is similar to that 

 of Tivela argentina, T. radiata, and T. gracilior. The two paits of 

 the hinge-plate in the right valve are similarly separated by a dee[) 

 groove or space, in front of wliich are three divergent cardinal teeth, 

 while behind it are a set of oblique and parallel plaits or ridges, 

 varying in number from three to ten. The true posterior cardinal 

 is a straight, narrow tooth, placed vertically under the umbo, and 

 connected at the top with the anterior margin of the valve. In some 

 specimens it is, moreover, visibly bifid or grooved. 



The pallial sinus in G. irreguhris is large, deep, linguiform, and 

 horizontal, extending to within a short distance of the anterior 

 adductor scar, but in G. difficilis it is smaller and shorter, not 

 reaching much more than half-way across the interior space. We 

 liave already seen (p. 267) that the depth of the sinus varies also 

 mucli in Tivela, and it is certain tliat the form of the sinus cannot 

 be taken as a character of generic or subgeneric importance. 



