MUSEUM OF COMPARATIVE ZOOLOGY. 231 



slender, and composed of a central more solid rod with a tube on each side of 

 it, and with its distal end enlarged in a somewhat hoof-shaped manner. From 

 the blood-vessel in the base a single tube issues to the root of each rod and 

 after continuing a short distance divides, one branch passing down one side 

 of the rod, the other crossing to the opposite side and continuing to join the 

 first again at the distal extremity ; it appeared as if the fluid passed down on 

 one side and back by the other. The exterior of the filaments is abundantly 

 ciliated, and though there is no organic connection, there is something in the 

 character of the surface of the knobs at the ends of the filaments which makes 

 them adhere tenaciously to each other or to any other part of the gill they 

 may touch; I could see nothing peculiar, but the mechanical effect showed 

 that something was there to produce it. The longest separate filaments were 

 5.0 mm. in length, the diameter of the stalk of the rods measured about 0.025, 

 and the knobs at the end 0.050 to 0.075 mm. The general outline of the gill, 

 as traced by the terminations of the filaments, differs in different individuals 

 to some extent. 



Habitat. Station 231 of Bartlett, in 1878-79, 95 fms., coarse .sand and rock, 

 off St. Vincent, bottom temperature 61°.5 F. Specimens on dead sea-urchin, 

 tests and on the thin marginal expansion of the shell of Phorus; Barbados, in 

 73 fms., coarse coral sand and broken shelLs, bottom temperature 70°.7 F.; 

 Station 134, off Frederikstadt, Santa Cruz, in 248 fms., coarse sand and broken 

 shells, bottom temperature 54°. 5 F. ; and Station 238, off the Grenadines, in 

 127 fms., fine coral sand, bottom temperature 56°.0 F. A fresh valve was 

 dredged by the U. S. Fish Commission, 36 miles S. ^ W. of Cape Hatteras 

 (Station 2601), in 107 fms., bottom temperature 64°.4 F. It will be seen from 

 the above figures that the species inhabits the warmer area. 



The form argentea above described appears closely similar to the figures of 

 D. Deshayesiana, given by Rouault. So nearly identical are they, that, until 

 I have had an opportunity of comparing the recent and fossil forms, I feel 

 barely justified in separating them. The figure of Ostrea temdplkata of 

 Seguenza * resembled Dimija so much in its exterior that I requested Pro- 

 fessor Seguenza to examine the interior and inform me of the character of the 

 muscular impressions. He did so, and also most kindly sent me two valves 

 which determine the correctness of my suspicions. The shell is an undoubted 

 Dlmya. It differs from Rouault's figures and from the recent argentea in 

 having a much stronger and coarser sculpture of divaricating radii, in its 

 shorter hinge-line and proportionally larger cartilage pit ; the visceral area is 

 smooth or slightly dotted, while in the recent form it is striated ; the relative 

 position of the muscular impressions and their form also differ somewhat in 

 the two shells. For these reasons, unless a considerable individual variation 

 may be hereafter demonstrated, it would seem that the two species may be 

 regarded as distinct. 



* Form. Terz. n. prov. Reggio, p. 123, t. xii. figs. 1, 1 a, 16, 1880. Miocene, 

 Aquitaniano, to Pliocene, Astiano, Calabria. 



