63 
This structure is most characteristic of Maccotjella, aud is visible on 
the interior of some weathered valves ; owing to varying resistance to 
weathering a more or less lenticular network is produced — nothing of this 
type has been noticed in Eurydesma. The nacreous character of the valves 
of Maccoyella is indicated at once by the sections, whilst the absence of any 
lustre in the sections of Eurydesma points to a suhnacreous variety, like the 
internal layer of Pinna, and the greater part of the test of the oyster 
fOstrea). On decalcification the whole of the test of Maccoyella is dissolved, 
hut in Eurydesma a small residue is left, without organic structure. The 
surface of the valv<^s in Eurudesnia is subject to exfoliation, and in only a 
few instances has the actual external surface been seen, and that not very 
])erfectly. '^I'he surface was coarsely laminate, and the laminae concentrically 
striate. Radii are generally absent. Morris described the surface of 
Eurydesma cord at a as “ radiately striate ” and “ indistinctly radiately 
striated,”^ and refers to Sowerby’s figure in Mitchell’s “ Three Expeditions, 
&c.”^ Dana says of the same species, “ marked with concentric lines of 
growth and low ridges and also Avith faint radiations.” In E. glohosa, 
Dana, E. elliptica, Dana, and E. sacculus, Dana, the author remarks that 
there is no radial marking. 
Only in the specimens referred to above have we met with any traces 
of radii — in relatively few cases have true external surfaces been seen. 
Within the lunular depression and on the anterior slopes the edges of the 
laminae are very coarse, roughening the surface. 
' Morris in Strzelecki — Loc. cit., p. 276. 
* Sowerby in Mitchell — Loc. cit., PI. II, fig. 1. 
E 
