184 IBLA CUMINGII, 



the body : the terga project about twice as much as the 

 scuta, and their flat apices generally diverge a little. The 

 tips of the valves are frequently broken off; their surfaces 

 are plainly marked or ribbed by the layers of growth, 

 which are wide apart. The bases of the valves externally 

 are hidden by the long spines of the peduncle. 



Scuta. — These are shorter and broader than the terga ; 

 their internal (PL IV, fig. 8 b') growing or corium- covered 

 surfaces are slightly concave, triangular, with the basal 

 margin longer than the other margins and slightly ex- 

 cised in the middle : there is no depression for the strong 

 adductor muscle : the internal surface of the free horn- 

 like portion, has a small central fold (formed by an oblique 

 crest) running from the summit of the triangular growing 

 surface to the tip of the valve : in perfect specimens, the 

 growing and the free horn-like portions (the latter repre- 

 sented much too long in fig. 8 a and b') are about equal 

 in length : the basal portion of one side of the scutum 

 overlaps the tergum. 



Terga. — The internal growing surface (fig. 8 b') is 

 almost diamond-shaped, and less in area than the scuta : 

 external surface rounded ; internal surface of the free 

 horn-like portion, slightly concave. 



Colour and Structure of Valves. — The external surfaces 

 of the scuta and terga are yellow along the middle, plainly 

 marked by zones of growth, and finely ribbed longitudi- 

 nally: the internal surfaces and sides of the horns of the two 

 valves, are coloured fine blue or purple ; in the terga, how- 

 ever, the internal surface is mottled with yellow. In some 

 specimens, especially in one from Tavoy, each zone of 

 growth was only very narrowly edged with blue. When 

 a thin layer is removed from one of the valves, the dark 

 blue or rather purple appears by transmitted light a 

 beautiful pale blue j and it is a very singular fact, that 

 this blue portion is permanently turned by very gentle 

 pressure into a fiery red ; the same singular effect is 

 produced by muriatic and acetic acids. This blue part is 

 much harder than the yellow; the latter exhibits, under 



