JUKHS-BROWNE : O.N TIVELA AND GRATELOUPIA. 269 



very rugose, and roughly divided into two ridges by a longitudinal 

 groove. 



In 2\ planulata (Brod. & Sow.) and in 1\ hinns (Phil.) the nymphal 

 plate is very deep, and there are two ridges on the inner side of it in 

 the riglit valve and two smaller ones on the left valve. 



T. tiiibila (Gray), T. madroides (Born), T. trigonella, and T. laevigata 

 (Gray) have hinges like that of T. intermedia. In mactroides the 

 rugose area of the right valve (Fig. 2) has a deep central groove 

 and a second shallow one under the base of the ligament, so that 

 there is an appearance of three parallel teeth. In the left valve 

 there are two narrow ridges corresponding with these grooves, and 

 the posterior cardinal is only united to the nymph at the top. 



In T. radiata (Sow.) and 2\ gracilior (Sow.) Ave seem to have the 

 ultimate phase of this line of development, for in these species, 

 which are quite distinct from one another, the cardinal plate has the 

 appearance of carrying six cardinal teeth. In T. radiata (right valve) 

 the true posterior cardinal is actually nearer the anterior than the 

 posterior end of the plate, while tlie first accessory tootli is a well- 

 developed dentiform ridge separated from the next by a deep groove, 

 except at the top, where it merges into the nymph ; the second is also 

 a long narrow ridge, and the third a shorter and less elevated one. 

 The anterior cardinal is small but visible just inside a bulge of the 

 lunular margin. In the left valve there are two well-developed 

 accessor)' tooth-ridges behind the normal posterior cardinal, which 

 is separated from them by a deep groove. 



In T. gracilior the dentition is similar, but there is a wider space 

 between the normal and the supplementary teeth of the right valve, 

 so that the different origin of the two sets of teeth is verj' clearly 

 seen. Tlie interspace extends right up to the umbo, the posterior 

 cardinal is united to the lunular margin, while the accessory teeth, 

 are obviously ridges on the nymphal plate, ])roduced sim[dy by the 

 grooving or channelling of that plate. The hinge-plate is thus 

 clearly divided into two parts, and there is a complete break or 

 discontinuity between the anterior margin of the shell and the nymph 

 or ligamental plate. In the left valve, liowever, this separation is 

 not quite so complete, because tlie posterior cardinal of that valve 

 is attached to the top of the nymph, springing, in fact, from the 

 posterior and not from the anterior miirgin. 



Fiuidly, in T. argentina (Sow.) we have a rather different develop- 

 ment, for in this species the nymph is not grooved longitudinally, but 

 obliquely, and only on a portion of its surface (Fig. 3). Its anterior part 

 is sculptured into four or live short oblique ridges, which are roughly 

 but not very regularly parallel to one another, while tlie posterior 

 part of the area is smooth. These oblique ridges, though less like 

 teeth than the strongly developed ridges of T. gracilior (Fig. 4), are 

 really more like those of Gratehupia than is the case in any other 

 species of Tivela. All three of the normal teeth are clearly developed, 

 and the rijiht posterior is strongly united to the end of the anterior 

 margin, but the left posterior is adherent to the thickened nymph 

 of that valve. 



