66 
is a similar dorsal gape, and, possibly, therefore a retiring resilium existed ; 
the byssal sinus is anterior to the right iimho, and the mantle retractor scars 
anterior to the adductor. Too little is known of the condition of this 
adductor scar in Eurydesma to show whether it he single or double, but in 
MaccoyeUa it Avas certainly double in some indi\dduals. The details of the 
articulus of 3Iaccoyella more closely resemble those of Eurydesma than they 
do Meleo.grina. 
(3) Aucella . — Dr. Waagen compared the hinge mechanism of his 
Eurydesma ylohosnm to that of Aucella. He suggested that what is here called 
the “ cardinal fold ” in the right valve of Eurydesma is analogous to the small, 
deeply divided-off AAung of the right A^alve of Aucella. It may be the case, 
but in the left valves of the two genera there is a further similarity — for in 
Eurydesma it sometimes happens that the anterior end of the left cardinal 
fold, as a small rounded projection, protrudes against the right valve. In the 
Australian Cretaceous Aucella hughendenensis, Eth. sp., there is a similar 
inward inflection of the margin of the left valve. 
Professor E. J. Pompeckj has recently made a most detailed 
examination of the genus Aucella.^ Owing to the lack of specimens of the 
exotic species, Ave are unable to follow his observations as closely as Ave 
could wish, but there appear to be certain agreements in the hinge-structure 
of Eurydesma and Aucella — (1) absence of an anterior adductor scar ; 
(2) absence of teeth ; (8) presence of a AA^ell-marked hyssal groove in the 
right valve ; (4) presence of a resilium [termed ligament by Pompeckj] ; 
(5) a scythe-shaped resilifer behind, in the umhonal regions in some forms. 
On the other hand, the cardinal margin appears to gape as in 
Meleagrina ; the resilium would therefore be a retiring one, and there is a 
highly differentiated small auricle in the right valve, and the hyssal sinus is 
anterior to the umbo in the right valve instead of posterior. 
It is considered that a reference of Eurydesma to either the Tridacnidae 
or Myalinidae may be dismissed Avithout comment, and after an exhaustiA^e 
compaiison Avith modern forms, tlie opinions of Morris and AVaagen are 
supported, and the genus placed in the Pteriidie, and close to Meleagrina, the 
Pearl Oyster, as the nearest living form, and Aucella amongst extinct forms. 
AVe consider Eurydesma to be an offshoot from the Pteria stock, greatly 
restricted in its geological and geographical range. 
’ Pompeckj — Zeit. deuts. Geol. Gesellsch., 1901, XIV, p. 219 et seq. 
