PWODUCTUS 109 



Description. Shell of medium size or somewhat larger, broader than 

 long, the hinge-line shorter than the greatest width. The dimensions of 

 a nearly complete specimen are : length from hinge-line to front margin 

 28.5 mm., length from umbonal region to front margin 33 mm., length 

 of hinge-line 29 mm., greatest width 36 mm., convexity of pedicle valve 

 22.5 mm. 



Pedicle valve gibbous, arched from the umbonal region to the front 

 margin with the curvature shorter and a little more abrupt posteriorly, 

 the umbonal region rather broadly pointed towards the beak and pro- 

 duced prominently beyond the hinge-line, the median portion of the valve 

 broadly flattened from the umbonal region to the anterior margin, this 

 surface curving abruptly into the lateral slopes, distinctly flaring towards 

 the lateral and anterior margins, the auriculations of moderate size and 

 not abruptly differentiated from the lateral slopes ; mesial sinus obsolete ; 

 beak strongly incurved. Surface marked by strong, rounded, radiating 

 costa? which divide frequently upon the posterior slope, rarely or not at all 

 upon the anterior and lateral slopes, until approaching the flaring margins, 

 where bifurcations again occur, the size of the costae upon the anterior 

 slope is usually about 1 mm. or somewhat more from center to center; 

 crossing the radiating costaj upon the posterior slope of the shell are more 

 or less conspicuous, concentric, wrinkle-like markings which give to that 

 surface a distinctly semireticulate appearance; the entire surface, when 

 well preserved, covered by faint, concentric lines of growth. Spine bases 

 usually inconspicuous, when best preserved they are scattered somewhat 

 sparsely over the anterior and lateral slopes, with sometimes, and perhaps 

 always if well preserved, a row along the cardinal margin. 



Brachial valve gently concave in the visceral region with the auricular 

 portions rather broad but not sharply defined, anteriorly and laterally 

 the curvature becomes greater as the surface passes into the produced 

 portion lying in contact with the opposite valve; the mesial portion of 

 the valve, from near the middle of the visceral region, is usually slightly 

 raised above the surface on either side in a slight, rather broad, ill- 

 defined mesial fold. Surface marked by radiating series of somewhat 

 elongate pits near the beak, which are interrupted intercostal furrows, 

 but the markings quickly change into distinct and rather strong radiating 

 costae similar to those of the opposite valve, which increase by bifurcation 

 and intercalation ; the visceral portion of the valve crossed by concentric 

 wrinkle-like markings, and the entire surface crossed by fine, concentric 

 lines of growth which are much stronger than those of the opposite valve. 



Remarks. Miller interpreted an external impression of a brachial valve 

 of this species as a pedicle valve of his species P. blairi, and illustrated 

 it as one of the types of that species. The specimen figured by Miller 

 as an external impression of the brachial valve of P. blairi is properly 



