- 203 - 



moderately inflated; right valve usually less inflated than the 

 left; dorsal margins divor-ing at an angle of 90° or less; both 

 valves sculptured with tv/ent^; to twenty-five slender, square to 

 suhroundFil radial ribs, se-of^rated "bj rsrv^vier interspaces; in the 

 interspanes and ocoasionally overridi.vg the ribs fine, inconspicu- 

 ous, slightly raised concentric lines present, rarely causing a few 

 anterior anri nosterior ribs to be scabrous; subniarglns v;ithout 

 radial sciiLpture or v/ith fine radiels, f ormiiig a continuous series 

 with those on the r!isk; hinge long, auricles decidedly uiiequal, 

 esoecially on the ri?ht valve; byssal notch deep and sharply sinu- 

 ate I; both auricles sculptured with irregular radial ribs and con- 

 centric lines,, the sculpture of the anterior auricle coarser and 

 more decidedly imbricated. 



Dimensions .- A right valve: 1st. 11. 8 mm.; alt., 1? mm.; 

 diam,, 1. 9 ram.; length of hinge, 7. 5 mm. A left valve: lat. 

 12 nim. ; alt., 1^. mm..; ''iam. 2.1 mm.; length of hinge, 7.2 mjn. 



Hema rk s . - The small, thin, delicately sculpturet^ valves of 

 this species are very abundant and the individuals range in stages 

 of gro77th from mature to very yoking. This abundance and the con- 

 stancy of '"he above note1, marked differences between it and the 

 recent P. omatus Lamarck (l) testify to the advisibility of rais- 



(ll. Anim. sans Vert., vol. 5, p. 176, 1819; Bruguidre, .:nc. 

 H4thod, pi. 214. fig. 5, 1739. 



