AVICULII)^. 281 



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,'Rsi[nc. Fossil; 

 Paris Basin. 



Malleus, Lam., 1799. 



Hammer-oyster. Syn. — Himotopoda, Schum., 1817- 



Distr. — 6 sp. China, Australia. None fossil. M. vulgaris, 

 Linn, (cxxxi, 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 (cxxxii, 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 margin ; 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 

 la3^er of the shell pearly, the inner porcellanous, hinge with a 

 pit like Hinnites. Branchiae consisting of long disunited filaments 

 attached to a cord-like band. 



Nayadina, Munier-Chalmas, 1863. 

 Distr. — N. Heberti, M.-C. (cxxxii, 82-84). 



Shell resembling a transverse Yulsella, 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 

 19 



