AVICULID A. 981 
Shell oblong, striated, subequivalve, with an inner pearly and 
outer fibrous layer; umbones straight, earless. Often found 
imbedded in living sponges. 
? VULSELLINA, Raincourt, 1876. V. Chaussyensis, Rainc. Fossil; 
Paris Basin. 
MauL.evs, Lam., 1799. 
Hammer-oyster. Syn.—Himotopoda, Schum., 1817. 
Distr.—6 sp. China, Australia. None fossil. IM. vulgaris, 
fina (exxxi, 74). 
Shell subequivalve, rugose, dorsal margin long, produced into 
a narrow wing on each side, median portion long and narrow, 
waved ; hinge with an elongated conical oblique cartilage-pit and 
no teeth. The young shell is like an ordinary Avicula, with a 
deep byssal notch in the right valve. 
Dimya, Rouault, 1850. 
Distr.—Several sp. Eocene to Pliocene; France. One living 
sp., W. Indies. D. Deshayesiana, Rouault (exxxii, 80, 81). 
Shell suborbicular, inequivalve, valves flattened—one of them 
(the left?) is believed to be adherent—thin; beaks small, sub- 
central, internally with a small triangular cartilage-pit in each 
valve; a finely denticulated line issues from the beaks and 
continues all round near the margin; it was evidently produced 
by the serrated edge of the mantle; muscular scars two, one 
anterior and one posterior; the latter is the larger, and both are 
situated some distance from the maryin; pallial line entire. 
This is a very peculiar shell; its form and structure resemble 
Placenta or Placuna, but there are no hinge-teeth present; the 
two muscular scars separate the genus from all Ostreacea, yet 
the figure is most like an oyster, and the ‘second adductor 
impression,” on account of which it is named Dimya, is rather 
like the small anterior scar in Pecten. 
A living species was recently dredged off the Antilles by the 
U.S. Coast Survey steamer Blake, attached to dead shells. It 
is practically identical with Ostrea tenuiplicata, Seguenza. It is 
essentially an oyster having two adductor muscles, the exterior 
layer of the shell pearly, the inner porcellanous, hinge with a 
pit like Hinnites. Branchiz consisting of long disunited filaments 
attached to a cord-like band. 
Nayaptna, Munier-Chalmas, 1863. 
Distr.—N. Heberti, M.-C. (cxxxii, 82-84). 
Shell resembling a transverse Vulsella, rostrated posteriorly ; 
the internal fibrous layer wanting. Elongately oval, subequi- 
valve, tumid, solid, inequilateral, anteriorly narrowly produced,, 
beaks tumid, obtuse, ligamental pit interior, anteriorly produced, 
a small tooth in the right valve behind the pit corresponding to, 
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