JUKES-BROWNE : CKETACEOUS AND EOCENE TENEEID^. 161 



existence before the Miocene period, it is interesting to find that two 

 little shells of Oligocene age approach very closely to those which are 

 grouped under that name. They were regarded by M. Cossmann as 

 species of Tivelina, but they differ in several respects from the typical 

 forms of that group, and are unquestionably a link between Tivelina 

 and Meretrix, so that they might be placed as a subgenus of either. 

 Since they have hitherto been referred to Tivelina I retain them in 

 that genus, but give them the distinctive name of Meretrissa to 

 indicate their affinities. 



The shells I refer to are Cytherea depressa, Desh., and C. Stampinensis, 

 Desh. Deshayes' figures of the latter species are not good, and 

 M. Cossmann informs me that the specimens from Pierrefitte, described 

 by Stan. Meunier^ under the name of C. dubia, are really identical 

 with Deshayes' Stampi?iensis ; I am also indebted to him for examples 

 of both depressa from Jeures and of Stampinensis from Pierrefitte, 

 thus enabling me to compare them with the species of Tivelina which 

 he had previously sent me. 



These shells are very small, depressa being only 8 mm. in transverse 

 width and 7 mm. in height, while Stampinensis varies somewhat, but 

 is often only 7 by 6 mm. They are subtrigonal, approximately equi- 

 lateral, and have a smooth outer surface marked only by lines of 

 growth. 



From Tivelina they differ in being much shorter transversely, and 

 consequently more equilateral and trigonal ; the teeth are more widely 

 and equally divergent, the right anterior cardinal sloping forward so 

 as to point toward the pit of the anterior lateral ; in the left valve the 

 anterior lateral is longer and comes closer to the anterior cardinal. 

 Again, the right posterior cardinal is not bifid, but entire or only 

 slightly grooved as in some recent species of Meretrix; while the 

 median teeth of both valves show a tendency to become bifid. 



From Meretrix they differ in their very small size, in the approxi- 

 mation of the lateral to the cardinal teeth, and in having a distinct 

 pallial sinus. M. depressa, which may be taken as the type, is rather 

 a thin shell, with a weak hinge, and is thus very different from the 

 solid recent forms of Meretrix ; M. Stampinensis is rather stronger, but 

 the hinge-plate is still weak and narrow. In the latter the sinus is 

 small and placed high up by the adductor scar ; in depressa it is rather 

 deeper, but does not reach the middle of the shell ; in both it is 

 obtusely rounded. 



8. DoLLFUsiA, Cossmann. 



This was proposed as a genus in 1886, but as it was based on 

 a single right valve, and no other specimens have been found, it 

 is impossible to be sure whether this valve is a normal one or 

 a malformation of some other shell. 



In the first place it is very small, only 6 mm. in length ; secondly, 

 it differs in shape from all other known members of the Veneridae, 



' Nouv. Arch, du Museum, 1880, p. 243, pi. xiii, figs. 17, 18; see also Cossmann 

 and Lambert, Mem. Soc. Geol. France, 1884, p. 82, pi. i, fig. 18. 



