RHYNCHOTETRA 207 



Description. Shell above medium size, strongly plicated, subtriangu- 

 lar in outline, cuneate posteriorly, longer than wide, the greatest width 

 anterior to the middle. The dimensions of the most perfect of the type 

 specimens are : length of pedicle valve 42 mm., length of brachial valve 

 40 mm., greatest width 38.6 mm., thickness 27 mm., width of sinus in front 

 27.3 mm. 



Pedicle valve arched from the beak to the front, the curvature becom- 

 ing much more abrupt anteriorly, the surface gently convex transversely 

 in the median portion of the valve, the gentle convexity extending to the 

 lateral margins in the widest portion, the postero-lateral margins ab- 

 ruptly deflected towards the opposite valve, the deflected portion on each 

 side forming one-half of a flattened or concave oval region which is pres- 

 ent on each side of the beak, and upon which the plications are obscure ; 

 mesial sinus obscure, obsolete in the posterior half of the valve, becoming 

 slightly depressed anteriorly and produced in a broad and rather long 

 lingual extension ; beak small, pointed, scarcely incurved ; the plications 

 strong, mostly simple, rounded or subangular on top, separated by broad 

 rounded furrows of which from 16 to 18 are present upon the valve, about 

 6 or 7 occupying the mesial depression near the front of the valve. Inter- 

 nally the dental lamella? are well developed and extend anteriorly from 

 the bsak for nearly one-third the length of the valve with only slight di- 

 vergence, they each join the inner surface of the valve, not being united 

 to form a spondylium elevated upon a median septum. 



Brachial valve more convex than the pedicle, the greatest convexity 

 towards the front, from this point of greatest convexity the surface 

 slopes to the beak with a long, gentle convex curvature, becoming a little 

 more abrupt near the beak, the anterior slope is much shorter, towards 

 the antero-lateral margins the surface curves much more abruptly, and 

 towards the postero-lateral margins it is abruptly deflected to meet the 

 similar deflected portion of the opposite valve ; mesial fold ill-defined and 

 obscure, not elevated above the general surface of the valve in the pos- 

 terior portion of the valve> but more distinctly elevated in front; beak 

 pointed and incurved beneath that of the opposite valve ; plications en- 

 tirely similar to those of the opposite valve. Internally a median septum 

 extends anteriorly from the beak for nearly one-third the length of the 

 valve. 



The finer surface markings consist of minute, radiating strise, and ex- 

 ceedingly fine concentric markings, in addition to which are occasional 

 stronger, concentric lines of growth, the finer markings usually being 

 visable only with a lens. 



Remarks. This species is characterized by its nearly simple plications 

 and its rather broad, triangularly subovate outline. The types of the 

 species are three in number, one nearly complete example with both 



