JUKES- BROWNE : OX TIVELA AND GRATEhOUPIA. 267 



A distinctive character of the Tivela liinge is the forward position 

 of the anterior cardiiialsin both valves, that of the left being in line 

 with the long compressed anterior lateral, so that these two teeth 

 liave the appearance of being projections from one long continuous 

 ridge ; that of the right valve is always placed so close to the anterior 

 margin and is moreover so low and slender that it maj' easily be 

 overlooked, and in some species it does become obsolete in adult 

 shells, though it is always discernible in young specimens. 



In most species of Tivela the pallial sinus is short and small, but 

 in T. ponderosa \t is deep and wide, reaching horizontally about half- 

 way across the shell and impinging on the pallial line below. The 

 size and depth of the sinus vary much in different species, and it should 

 not be described as always small. 



T. damaoides, Gray, from Peru, has the same hinge-characters as 

 T. ponderosa, and the specific differences are so small that it may be 

 regarded as a mere variety of the latter. In the right valve of 

 damaoides, however, the inner edge of the nymph is raised into 

 a narrow tooth-like ridge (Fig. 1). 



T. bicolor, Gray, from West Africa, has a similar hinge, the nymph 

 of the right valve being nearly flat and only slightly rugose; so also 

 are those of 2\ tripla (Linn.) and T. dolahella (Sow.); 1\ polita 

 (Sow.) has a similar flat rugose nymph, and is remarkable for its 

 deep pallial sinus, which extends more tlian half across the shell and 

 obliterates part of the pallial line. 



In 2\ stuUorum (Mawe), which is better known as tlie T. crassa- 

 telloides of Conrad, tliere is a further development of this ridge, the 

 whole thickness of the nymph in the right valve being produced or 

 raised into a rugose dentiform ridge which forms a supplementary 

 tooth, but is clearly only a thickened development of the nymph. 

 This fits into a rugose trough or space between the posterior cardinal 

 of the left valve and a narrow ridge or plate on the nymph 

 above it. 



T. natalensis (Dunker) is interesting because it shows what seems 

 to be the first stage in the development of two nymphal teeth. The 

 nymph is not thickened as in cransatelloides, being still low and 

 flatfish, but is grooved or channelled along the median line, and the 

 left valve carries a narrow tooth-like ridge Avhioh fits into this 

 groove. The hinge thus presents the appearance of having four 

 cardinal teeth in each Talve, for the grooved nymph might be 

 mistaken for a bifid posterior tooth. 



In this species also it is noticeable that there is a rather wider 

 space between the nymph of the right valve and the true posterior 

 cardinal, so that the latter is shorter and more central in position 

 than is the corresponding tooth in the species previously mentioned ; 

 it is a straight, narrow, entire tooth, very different from the elongate 

 grooved or bifid posterior of Meretrix, and it is united at the top, 

 under the umbo, to the end of the lunular margin of the shell, as in 

 Jleretrix and other genera of Veneridte. 



In T. compressa (Sowerby) the nymphal plate is broader, and there 

 are several parallel rugosities, the inner one of which becomes in 



