PANAMIC-PACIFIC PELECYPODA 467 



U.S.S. "Albatross," station 3392, Gulf of Panama in 1270 fathoms, 

 hard bottom, temperatures 36.4° F. (Dall, 1908.) 



Subgenus DERMATOMYA Dall, 1889 

 Type species by monotypy, P. (D.) mactroides Dall. Recent, off the 

 coast of southern Chile. 



Surface smooth, without granulation, the pallial sinus distinct, 



Poromya (Dermatomya) equatorialis Dall Plate 83. figure 8 



Poromya {Dermatomya) equatorialis Dall, 1908, Bull. Mus. Corap. Zool., vol. 43, No. 6, 

 pp. 429, 430, pi. 5, figs. 1, 2 Gulf of Panama in 1672 fathoms and also off the 

 coast of Ecuador in 741 fathoms. 



Shell subtrigonal, inflated, slightly inequivalve, white covered by a 

 strong dark olive-gray periostracum, paler near the basal margin. Surface 

 smooth except for lines of growth, without granulation or surface striation; 

 form as figured; interior bluish white, slightly pearly; ligament and resilium 

 combined, short, set obliquely, internally, under and behind the beaks; 

 there is no lithodesma; hinge comprising one stout cardinal prominent in 

 the left valve entering a socket in the opposite valve in front of a long, 

 nearly horizontal lamina, which forms the base of the resiliary pit; there 

 is also one strong posterior left lateral, and a feeble anterior one on the 

 same valve, which last is hardly to be distinguished from an angularity of 

 the hinge margin; pallial sinus wide, rounded, rather short. Length of shell, 

 18, alt. 15, max. diam. 5+6.5 mm., the smaller dimension being that of the 

 left valve, which basally fits inside the other and has a slightly tortuous 

 margin behind. 



This species, besides its much greater size, has beaks more inflated 

 and higher, and the posterior end blunter than we find in P. mactroides 

 (Dall, 1908). 



Family LTONSIIDAE 



Shell generally elongated, oblong, thin or heavy, nacreous, regular or 

 markedly distorted in some nestling forms. Hinge edentulous. Ligament 

 internal, the resilium seated in a long, narrow, resilifer under the dorsal 

 margin behind the beak, further reinforced by a large shelly plate or 

 lithodesma. Test smooth or granulose. 



Genus LTOK"SIA Turton, 1822 

 Type species by monotypy, My a striata Montagu, 1822 (= My a 

 norwegica Gmelin, 1790). Northern European seas. 



Shell generally elongate, regular, inequivalve, the posterior side longer, 

 more or less flexed and gaping at the end, the texture thin, nacreous, the 

 surface covered with a thin, inconspicuous, yellow or gray periostracum, 

 smooth, concentrically marked or with fine radial threads. Hinge edentulous, 

 the ligament internal, the resilifer in the shape of an elongated, grooved 

 ridge under the dorsal margin behind the beak, similar in each valve. The 

 middle section of the resilium is covered by a large, calcareous plate or 

 lithodesma, the brown or fibrous portion of the resihum developed along 

 its sides or in the contact zone with the resilifer. 



Genus ENTODESMA POiilipipi, 1845 

 Type species by monotypy, E. chilense Philippi. 



Shell large, coarse, and of irregular shape due to a nestling habit. 

 Periostracum coarse. Lithodesma large. 



