PRODUCTUS 105 



more abrupt to the cardinal margin ; mesial sinus originating in the um- 

 bonal region, of only moderate depth and width, rounded in the bottom 

 and ill defined laterally. Surface of the valve marked by rounded, 

 radiating costse which increase by bifurcation and intercalation poster- 

 iorly, but anterior to the middle they usually continue without further 

 division to near the front margin, where they tend to become obsolete, the 

 distance between the costae varies from. 5 to 1.5 mm. in the anterior portion 

 of the valve, about 1 mm. being the usual distance from center to center, 

 posteriorly they are more closely crowded; on the posterior slope of the 

 valve the radiating eostas are crossed by distinct, rather regular, wrinkle- 

 like concentric markings, giving to that portion of the valve a distinctly 

 semireticulate ornamentation ; spine bases are either nearly obsolete upon 

 the surface of the valve or they are scattered irregularly upon the anterior 

 and lateral slopes, one or more sometimes being present upon the auricu- 

 lar extensions of the valve ; in front of the spine bases, when present, 

 two or more of the surface costae are sometimes united to form a broader 

 costa which continues to the anterior margin. Internal characters of the 

 pedicle valve not observed. 



Brachial valve gently concave in its posterior portion, flattened towards 

 the cardinal extremities opposite the auriculations of the other valve, 

 anteriorly and antero-laterally the valve is more abruptly curved and is 

 produced nearly in contact with the anterior and antero-lateral portions 

 of the opposite valve ; anteriorly the mesial portion of the valve is elevated 

 in a low, rounded, mesial fold to correspond with the sinus of the oppo- 

 site valve. Surface of the valve marked by radiating costaa which are 

 less distinctly developed than those of the opposite valve, and in the 

 posterior portion by concentric, wrinkle-like markings which are also 

 less distinct than those of the pedicle valve; no spine bases have been 

 observed upon this valve, although they may be present in some cases. 

 Internally the cardinal process is large and strong, its entire length 

 being produced posteriorly beyond the hinge-line into the protuberant 

 umbonal portion of the opposite valve ; it is bifid at its extremity and is 

 flanked at its base by a thickened ridge on either side which extends 

 laterally nearly parallel with and close to the cardinal margin, distally 

 these lateral ridges become broader and less differentiated from the 

 general surface of the valve, but the thickening extends to the cardinal 

 extremities ; anteriorly from the cardinal process a low, thin median sep- 

 tum reaches to near the middle of the valve ; on either side of the median 

 line where the valve begins to approach more nearly to the curvature 

 of the opposite valve, the inner surface is covered with fine, crowded, 

 spinule-like papilla? which are inclined anteriorly. 



Remarks. This species is characterized by the strongly convex pedicle 

 valve, the deep visceral cavity between the two valves, the usually small 



