160 PROCEEDINGS OF THE MA.LACOLOGIOAL SOCIETY. 



relationship. Probably the points wbicb he had in mind were the 

 somewhat triangular shape and the short pallial sinus, but there is 

 no resemblance in the hinge-characters. Of them he only says 

 that the hinge is distinct from that of Callista without specifying 

 the exact points of difference. 



Fischer ^ did not express any opinion on the precise relationship 

 of Tivelina, merely stating that Chionella and Tivelina were names 

 proposed for some fossil groups of Meretrix, and giving the following 

 brief description of the latter: "Forme subtrigone; surface lisse ; 

 un pli sinueux en arriere." The sinuous fold, however, does not 

 exist in all the species referred by M. Cossmann to Tivelina. 



Dr. Dall, in 1902, placed the group as a subgenus of Fitaria, 

 giving only a brief account of it, and stating the hinge to be 

 "as in Pitariar In his later memoir, however (op. cit., 1903, 

 p. 1265), he says "hinge as in Chionella,^^ but still retains the 

 group as a subgenus of Pitaria, an arrangement with which I 

 cannot agree. In any case it seems desirable to print a more 

 complete and diagnostic description of Tivelina, and as M. Cossmann 

 has kindly supplied me with specimens of eight of the principal 

 species, I have carefully examined and compared these in order to 

 express the distinctive characters which are common to all. 



Shell oval, compressed, anteriorly expanded and posteriorly 

 attenuated, frequently showing a shallow fold or depression which 

 slightly sinuates the ventral border. Surface smooth or finely 

 striated. Umbones small. Lunule long, lanceolate, and defined by 

 incised lines, but never impressed, and sometimes the area is gibbous. 

 Hinge-plate weak and narrow, elongated anteriorly and attenuated 

 posteriorly, so that the cardinal area is short. The teeth resemble 

 those of Callista, but are small and weak ; in the left valve are three 

 divergent cardinals, the middle being thickest, and a prominent 

 triangular anterior lateral ; in the right a narrow feebly- grooved 

 posterior, while the median and anterior cardinals are short and 

 nearly parallel, and in front are two narrow anterior laterals 

 bordering a deep, narrow, elongate pit. The pallial sinus is always 

 short and rounded. 



On the whole it appears to me that Tivelina is more nearly 

 allied to Callista than to any other genus. It differs, however, in 

 the absence of radial strise, in the umbones being small and nearer 

 the middle line of the shell, in the curiously short and narrow 

 hinge-plate, and in the narrowness of the right posterior cardinal; 

 also in the small rounded pallial sinus. Whether these characters 

 are of generic importance or not it is difficult to say, but they seem 

 of rather more diagnostic value than those which distinguish Pitaria 

 from Callista. 



Meeeteissa, n.subgen. PI. VI, Fig. 7. 

 Although the genus Meretrix, as restricted by Gray and Deshayes, 

 and typified by Meretrix morphina, does not seem to have come into 



1 Manuel de Conchyliologie, 1887, p. 1080. 



