NO. H39. nr.KP-WATEi; Moi.i.rst'A VKRHILL AXI> r.rsii. 777 



Family PHOLADIIKE. 

 MARTESIELLA, new subgenus. 



This subgeneric name is proposed for the following species, which 

 ditt'cis from Narlcsia in having a well-delined, elongated, median, dorsal 

 plate, posterior to the umbos, in addition to the shield-shaped one over 

 them. 



MARTESIA (MARTESIELLA) FRAGILIS, new species. 



(Plate LXXIX, liy. ID. i 



Shell small, white, thin, fragile, wedge-shaped. The anterior end is 

 very short and broadly rounded, the aperture nearly closed in our 

 largest specimen by a pair of callous plates. The anteio dorsal margin is 

 recurved toward the uinbos, but not appressed, and forms a deep, spiral, 

 open cavity. The valves have a very obtuse anterior emargiuation. 

 A broad and moderately deep sulcus runs from the beak to the ventral 

 margin; in front of this the surface is covered by thin concentric ribs, 

 which curve down waul at the sulcus and form a distinct angle in line 

 with the anterior emargination and corresponding with a slight ridge 

 on the surface; these concentric ribs are crossed by tine radiating lines, 

 which produce tine serrations on their edges. Posterior to the sulcus 

 the surface is marked only by irregular lines of growth, which, near it, 

 take the form of more distinct grooves or ridges. The posterior end is 

 prolonged, compressed, and bluntly rounded. The umbonal plate is 

 thick, relatively large, and usually heart-shaped, with the posterior end 

 broader and distinctly emarginate in the middle; the anterior end 

 tapers somewhat and is blunt and angulated, or sometimes subacute. 

 The posterior dorsal plate is long, narrow, and somewhat spatnlate or 

 clavate, and stands well in relief above the dorsal margin, with the 

 edges free and the narrow anterior end running under the posterior 

 end of the umboual plate. 



Length of one of the largest specimens, 7 mm.; height, 4..~> mm.: 

 thickness, 4 mm. 



. Young specimens 3 or 4 mm. in length are relatively shorter and 

 thicker than the larger ones, but even these have the anterior callous 

 pretty well developed; the umbonal plate is usually shield- shaped, 

 the lateral borders emarginate, in contact with the most prominent 

 part of the uinbos; the posterior border is distinctly emarginate. and 

 the anterior end has a central point or mucro, sometimes defined by 

 slightly concave posterior edges. 



Many live specimens were found in a piece of wood floating near 

 station 250(5, N. hit. 37 J', W. long. 68 8', 1885. 



