MUSEUM OF COMPARATIVE ZOOLOGY. 233 
such a purpose, has proposed a division based on the number of branchizx, 
those with two branchie on each side composing his order Tetrabranchia, as 
opposed to the Dibranchia with one gill on each side. But to this arrange- 
ment Dimya does not lend itself ; Ostrea, Mytilus, etc., to which it is certainly 
most nearly allied, having four branchial leaves, to say nothing of the addi- 
tional accessory plates which may be taken as representing a third pair. 
Moreover, some species of Arca (ex. A. ectocomata Dall) have but a single 
pair, while others have two or three. All the evidence points to the conclu- 
sion that the Pelecypoda comprise but a single order, knit closely together by 
inter-ramifying characters. 
The genus Dimyodon Munier Chalmas (1886), of the great Odlite, appears 
to differ from Dimya by the projection of the wrinkled hinge-areas so as to 
form striated teeth, recalling those of Plicatula, and by its single posterior 
adductor scar. It has not been reported in a recent state. 
Famiry AVICULIDZ. 
Genus AVICULA Lamarck. 
Avicula atlantica Lamarcr. 
Avicula sp. indet. Dall, Bull. M. C. Z., TX. p. 117. 
Habitat. Station 26, 116 fms. 
Two very young living specimens were obtained here, and worn fragments 
were found from other localities. It is spread over the whole Antillean region, 
and northward in suitable localities at least as far as Hatteras. 
Famitry MYTILID2. 
Genus MYTILUS Line. 
Mytilus exustus Lixyé. 
Mytilus exustus Linné, Dall, Bull. M. C. Z., IX, p. 117. 
Habitat. Sigsbee, off Havana, 158 fms.; Barbados, 100 fms., etc. 
This is spread over all the shores of the Antilles, and the specimens obtained 
from more than a few fathoms are drifted or disgorged by fishes into the deeper 
water. The species does not live in deep water. 
