550 Systematic Paleontology 



wards from the umbones to the posterior basal margin ; umbones small, the 

 apices acutely pointed and barely extending beyond the dorsal margin, 

 placed within the anterior fourth, shell abruptly compressed in front of 

 the umbones, thus forming a trigonal auricle, area behind the umbones 

 broader and less sharply differentiated, approximately a low isosceles 

 triangle in outline; posterior lateral margin a little longer than the 

 posterior dorsal in the adults, a little shorter in the young, showing at all 

 stages a broad and shallow byssal sinus; ventral margin approximately 

 equal and parallel to the hinge line; external surface smooth excepting 

 for incremental sculpture; ligament area narrow and much elongated, 

 produced almost to the distal extremities of the dorsal margin, widest and 

 feebly depressed in the umbonal region, more attenuated posteriorly than 

 anteriorly, finely striated parallel to the cardinal margin ; a single amor- 

 phous pseudo-cardinal developed in the left valve, springing from beneath 

 the hinge margin and received in a subumbonal socket in the right valve ; 

 a single obliquely pyrif orm adductor impression situated a little more than 

 half way down the oblique depression from the umbones to the posterior 

 basal angle, a smaller scar, probably the pedal, a little behind it ; a number 

 of minute byssal pits distributed along a line running from beneath the 

 umbones approximately parallel to the anterior lateral margin; pallial 

 line indistinct. 



Dimensions. — Altitude 40 mm.; length of hinge line 40.5 mm. ; length 

 from anterior dorsal margin to posterior ventral 60±mm. ; semi-diam- 

 eter 6 mm. 



Type Locality. — Brightseat, Prince George's County. 



The type of Pteria rhombica is a very striking shell. No other indi- 

 viduals occur which approach it in size, but a number of forms were col- 

 lected at the same locality which present adolescent characters and which 

 differ from the type only in having the posterior ventral margin a little 

 more produced relatively and in the much smaller size, running about 

 15 mm. between the opposite margins and 35 mm. along the diagonal. 



The species is remarkable for the regularity of the rhombic outline and 

 the broad and uniform inflation along the diagonal. 



