258 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



The concentric costse are rather obscure on the posterior part of the 

 valves; but on reaching the narrow groove extending from the beaks almost 

 directly to the base, a little in advance of the middle of the valves, the}' 

 become more distinct, and are deflected abruptly upward, nearly parallel to 

 the vertical margin of the angular hiatus, becoming, as they ascend, very fine, 

 closely-arranged, and delicately-crenated lines. In passing upon that part of 

 the valves extending out over the hiatus, these fine lines are abruptly deflected 

 forward and again suddenly enlarged, but continue to be very regular, and 

 rapidly diminish in size, and converge as they pass toward the anterior 

 extremity. 



The grooves left on the internal cast by the internal ridges of the valves, 

 are much as in the foregoing species, excepting that the one corresponding to 

 the vertical groove on the outside of the valves is much deeper, and regu- 

 larly punctate at the points where the concentric markings cross it, while 

 the posterior one is narrower, less oblique, and passes down in front of each 

 posterior umbonal slope, instead of behind and above it. Nearly all the space 

 back of the oblique posterior sulcus of the cast is occupied by a large leaf- 

 like muscular'? scar. The sinus of the pallial impression, as seen on inter- 

 nal casts, is narrow, nearly vertical, and ascends more than half-way up 

 between the posterior oblique groove and the vertical one in advance of it. 



Locality and position. — Same as last. 



Genus MARTESIA, Leach. 



Synon.—Martesia (Leach, MS.), Blaiuv. (1824), Diet. Sci. Nat., * \— Desh., ib., Ill, 421.— Sowerby, jr., 

 Conch. Man., 186.— Gray (1847), Zool. Proceed., 188, and (1851) Ann. and Mag. N. H., 

 VIII, 380.— H. and A. Adams (lb5(i), Gen. Recent Moll., II, 330.— Tryon (1862), Classi- 

 fication Plwhulidw, Proceed. Acad., 89; also (1873) Am. Marino Conch., 127. 

 Martresia, Gray (1842), Syuon. Brit. Mus., 91. (Typogr. error.) 



Etym.— ? 



Type. — M. striata (= Photas clavata, Lara.). 



Shell transversely ovate-oblong, equivalve, generally produced poste- 

 riorly ; anterior hiatus closed in .the adult by a shelly plate ; surface with 

 concentric sculpturing, crossed by a - single umbonal furrow; umbonal valve 

 single; dorsal and ventral margins with lanceolate accessory pieces, or valves. 



This genus will be readily distinguished from the last by its anterior 

 hiatus becoming closed by a shelly secretion in the adult, as well as by 

 wanting the posterior internal ridge of that group. If Turnus has no acces- 

 sory valve, as appears to be the case, that would be another strongly-marked 



