MUSEUM OF COMPARATIVE ZOOLOGY. 233 



such a purpose, has proposed a division based on the number of branchiae, 

 those with two branchiae 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 Area (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 Pelecyjjoda comprise but a single order, knit closely together by 

 inter-ramifying characters. 



The genus Dimyodon Munier Chalmas (1886), of the great Oolite, appears 

 to differ from Dimya by the projection of the wrinkled hinge-areas so as to 

 form striated teeth, recalling those of Plicatida, and by its single posterior 

 adductor scar. It has not been reported in a recent state. 



Family AVICULID^. 



Genus AVICULA Lamarck. 



Avicula atlantica Lamarck. 

 Avicula sp. indet. Dall, Bull. M. C. Z., IX. 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. 



Family MYTILTD.^. 



Genus MYTJLUS Linn:^. 



Mytilus exustus Lixne. 

 Myiilus exustus Linn^, 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. 



