STREPTORHYNCHUS 69 



median septum of the pedicle valve, from Schellwienella it may be distin- 

 guished at once by its biconvex shell and its irregular manner of growth, 

 its closest relationship being with Schuchertella. In both Streptorhynchus 

 and Schuchertella the median septum of the pedicle valve is absent, as are 

 also true dental lamellae, leaving then only the greater convexity of the 

 valves, their irregular manner of growth, the shorter hinge-line and the 

 form of the cardinal process as diagnostic characters of Streptorhynchus. 

 The distortion of the valves in Streptorhynchus, especially the pedicle 

 valve, is due to the attachment of the shell by the apex of that valve, while 

 Schuchertella is unattached. 



STREPTORHYNCHUS TENUICOSTATTJM n. sp. 

 Plate IV, Figs. 3-5 



Description. Shell of medium size or a little larger, wider than long, 

 the greatest width near the mid-length of the shell, subelliptical in out- 

 line, the hinge-line shorter than the greatest width, the cardinal extrem- 

 ities rounded. The dimensions of a nearly complete but somewhat 

 crushed example are : length of pedicle valve 28 mm., length of brachial 

 valve 24.5 mm., greatest width 30.6 mm., length of hinge-line 23 mm., 

 thickness 10 mm., height of cardinal area 3.5 mm. The dimensions of a 

 nearly perfect brachial valve are: length 27 mm., greatest width 39.5 

 mm., length of hinge-line 27 mm., convexity 6 mm. 



Pedicle valve convex, more or less distorted, depressed convex in the 

 umbonal region with an irregular contour towards the lateral and 

 anterior margins; mesial sinus obsolete; beak small, only slightly ex- 

 tended beyond the cardinal margin; cardinal area flat, of moderate 

 height, sloping posteriorly from the hinge-line at an angle of 40 degrees 

 or more, the delthyrium about as wide as high, covered by a convex 

 deltidium. Internal characters not observed. 



Brachial valve much more regularly convex than the pedicle, the great- 

 est convexity near or posterior to the middle, the surface more or less 

 compressed towards the cardinal extremities, the umbonal region not 

 protuberant beyond the cardinal margin ; mesial portion of the valve not 

 differentiated. Surface of both valves marked by radiating costse but 

 those of the two valves somewhat different in character. The costae of the 

 pedicle valve are very slender and angular, separated by broad, flat, in- 

 tercostal furrows, from one to three costas occupy the space of one milli- 

 meter, the new ones being added by intercalation at intervals at the middle 

 of the intercostal furrows; the costge are crossed by exceedingly fine, 

 somewhat wavy, concentric markings which are more conspicuous across 

 the flat intercostal furrows; the valve also marked by strong, irregular, 

 wrinkle-like, concentric markings. On the brachial valve the radiating 

 costse appear somewhat coarser because the intercostal furrows more 

 nearly equal the costas in width, although essentially the same number 



