VOL. XII, 

 1889. 



PROCEEDINGS OF THE NATIONAL MUSEUM. 291 



Maximum altitude of shell, 11.5; maximum longitude, 10.5 ; diameter, 

 9 nim . This form has almost exactly the outline and size of P. sublevis 

 Verrill, to which I at first referred it. But that species has the typical 

 teeth of Poromya, while in this the only tooth in tlie right valve is a 

 siugle slender spur-like cardinal, and the left valve is almost edentulous. 

 In its teeth this species agrees much more nearly with P. (Cetomya) 

 tomato, than with any of the normal species, aud, like that, has a twist 

 in the posterior rostrated part of the shell which I have not observed in 

 any genuine Poromya, and which is not mentioned by Verrill in his de- 

 scription of P. sublevis, of which I have not had an opportunity of see- 

 ing specimens. I can hardly (after seeing many specimens of Poromya) 

 believe that such differences in the hinge are not of specific value. 



Subgenus DERMATOMYA Dall. 

 Dermatomya Dall, Bull. Mus. Comp. Zool., xvm, pp. 449, 4f>2, Juue, 1889. 

 Dermatomya mactroides Dall. 

 Plate vin, Fig. 8. 



Poromya (Dermatomya) mactroides Dall, Bull. Mus. Comp. Zoiil., xvm, p. 449 June 



1889. 



Shell large, stout, strong, with a strong epidermis, olive gray toward 

 the beaks, paler, inclining to greenish, toward the margins in the adult; 

 epidermis raised into wrinkles on the posterior area and folding in over 

 the bisal margins; young shell with a few sparse granulations near the 

 anterior and posterior margins, adult without visible granulations, the 

 epidermis mostly shining and the shell showing iridescent through it; 

 the young are subrhomboidal, the adults have the beaks prominent, 

 high, subcentral; the anterior end rounded, the posterior very slightly 

 produced; surface sculptured only with more or less evident incremental 

 lines; luuule aud escutcheon are visible on a close scrutiny, though not 

 marginated by a line ; the former is cordate, the latter narrow and long; 

 hinge of Poromya, strong; ligament short, half internal; interior faintly 

 iridescent, pallial and muscular scars distinct but not emphatic; the 

 pallia! Hue is deeply and rather narrowly sinuated; the basal margin 

 is perfectly plain; altitude of adult shell, 16; longitude, 18; diam- 

 eter, 12 mm . 



Hab. — U. S. Fish Commission Stations 2781, 2783, and 2785, on the . 

 w-est coast of Patagonia, on a muddy bottom in 122, 348, and 449 fath- 

 oms; bottom temperature 4G°.9 to 49°.9 F. Also at Station 2793, in 741 

 fathoms, mud; bottom temperature 38°. 4F. off the coast of Ecuador. 



The superficial resemblance to a small Mactra presented by this shell 

 needs no further comment. It is sufficiently evident. 



This fine species differs from the typical form of the genus in the 

 absence of the superficial granulations, and in the presence of a deep 

 and strong pallial sinus, which characters indicate that it should form 

 a special section of the group. The hinge is also remarkably coarse 

 and strong. 



