T R I D A C N A . 



Plate VI. 



Species 5. (Fig. 5 a; and PI. VII. Fig. 5 b, c.) 



Tridacna compressa. Trid. testa, ovatd, eompressa, sex- 

 costaid, costis latii/sculls, radialim Uratis, dense squa- 

 matis, squamis parvuUs, erectis, costarum mlerstitiis 

 radialim Uratis, liris tribus centralibus prominentiori- 

 liis ; alba, inlerdmn pallide cilrino tindd. 



The compressed Tridacna. Sliell ovate, compressed, 

 six-ribbed, ribs ratlier broad, radiately ridged, densely 

 scaled, scales ratlier small, erect, interstices between 

 the ribs ridged, the three central ridges being the 

 more prominent ; wliite, sometimes tinged with pale 

 lemon-coloui'. 



Hab. •? 



Either this is a species or it is a non-elongated form of 

 T. elongata. In a genus like Tridacna, where the species 

 are so numerous and the varieties so variable, some licence 

 in naming is really usefid to distinguish them. In the 

 pi-esent instance, the shell is broader from the umboes to 

 the margin and more compressed than that of the typical 

 T. elongala, and the scales are smaller in proportion to the 

 width of the ribs, more distant, and more erect. All this, 

 it may nevertheless be argued, is a correlation of growth 

 arising out of the compression of the valves. At Plate 



VII., Fig. 5 4 and .5 c, are represented younger specimens 

 of the same form. 



Species 6. (Mus. Cuming.) 



Tridacna serkifera. Trid. tesld late ovatd, compressiiis- 

 culd, temiiculd, iri- qnadri-costatd, costis interstitiis- 

 que amplis, nndis, radiatim sulcato- stria tis, striis in 

 interstitiis Jortioribus, marginibns anticis sqnamoso- 

 serratis ; alba vel pallide citrind. 



The serrated Tridacna. Shell broadly ovate, rather 

 compressed, somewhat thin, three- to four-ribbed, 

 ribs and interstices large, scaleless, radiately groove- 

 striated, striee stronger in tlie interstices, anterior 

 margins scale-serrated ; white or pale lemon-colour. 



Lamarck, Anim. sans vert. vol. vii. p. 11. 



Hah. Moluccas. 



The character of this species is to be without scales, 

 except on the anterior margins, where they are short and 

 often nearly obsolete. But it not imfrequently happens, 

 as in the specimen figured, that all the riljs are serrated 

 with minute rudely-developed scales in the vicinity of the 

 umboes. The shell is very copiously groove-striated 

 througliout, and the striae arc much stronger in the inter- 

 stices between the ribs than upon tliem. 



.June, 1802. 



