62 
Morris descril)ed Eurydesma as thick at the umhones/ and Dana as 
very much thickened under the umbo of the left valve; he gave a transverse 
section acioss a valve of E. sacculns showing a thickness of nearly one and a 
quarter inches.’^ Waageii refers to the “ enormous thickening of the shell 
substance in the apical region.”^ 
The structure of the test in the Pteriidse is prismatic and nacreous, 
“ the exterior heiiig composed of prismatic cellular substance, and the interior 
of true nacre. We have been unable to detect the prismatic cellular layer 
in Eurydesma ; this may he due to the liability to exfoliation. A section 
transverse to the dorso-ventral axis taken completely through the ventricose 
umhonal region and articulus distinctly shows the successive laminse of the 
test, the laminm broadening as the hinge is receded from. Dr. John Quekett 
rigured this laminar structure in the Pearl Oyster,^ as well as the outer 
prismatic layer. 
The test of Eurydesma has undergone considerable alteration, but, 
notwithstanding this, tangential sections taken at varying depths display 
near tlie surface a structure resembling sojiiewhat the peculiar hackly, 
laminar structure sliown in Dr. Carpenter’s figure of polished nacre® ; these 
roughened undulations are probably the “plications of the animal basis of 
the shell.”” There is no cancellated structure in the test of Eurydesma as is 
the case in most of the Spondyli, “ the office of the cancelli being to render 
the shell lighter.”® 
Very different is the shell structure of Maccoyella, another member 
of the Pteriidm, and although allied to Eurydesma, more nearly so to 
Meleayriua. A section of the test of Maeeouella taken parallel to the 
general surface, whetlier external or internal, aff ords a most pearly lustre, 
and shows a seri('s of fusiform or lenticular figures defined only by a difference 
in colour. Under ])olai ized light the shell sections of both genera give the 
same reaction, probably that of calcite. 
' M( i ris in Strzelecki — Op. cit., p. '275. 
^ Dana in Wilkes — A-'C. ciO, p. 699, PI. VII, fig. SrZ. 
’ Waagen — .Salt Range Fossils, Zoc. ciV. , p. 199. 
* \V B. Carpenter — Kept. Brit. Assoc., I (1844), 184.5, p. 20. 
* Quekett — Lectures on Histology, 18o2, p. 29', fig. 182. 
'Carpenter — Loc. cit., PI. VIII, fig. 17. 
’ (Quekett — Loc. cit,., p. 301. 
^ Quekett — Loc cit., p. 300. 
