ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND *BA^SLER 39 



reticulate ornament of the shell somewhat coarser than that of W. 

 limatula and a grade finer than in W. scitula and W. breviuscula. 



Dimensions of the type specimen: Greatest length 3.8 mm., greatest 

 preserved height 2.0 mm., probable height of perfect valve, about 

 2.3 mm., thickness of single valve about 0.9 mm. 



Remarks. — Despite the imperfections of the unique example upon 

 which this species is based, its preserved characteristics seemed suffi- 

 ciently peculiar and indicative of the complete form to justify its 

 description. If correctly understood, the species is allied to W. 

 scitula and W. ohsoleta, being in fact an intermediate form. Better 

 specimens of all three of these species are necessary before their rela- 

 tions may be accurately determined. 



Occurrence. — Sandstone in Bright Angel shale, Bass Canyon, Grand 

 Canyon of the Colorado, Ariz. 



Holotype.— Cat. No. 56486, U.S.N.M. 



WALCOTTELLA OBSOLETA, new species 



Plate 4, Figure 13 



Description. — Only a single specimen, thought to be a right valve, 

 is known of this species. The curvature of the dorsal and ventral 

 slopes is unusually abrupt. This fact, together with apparent im- 

 perfections of both the posterior and anterior thirds of the cardinal 

 edge, suggests that the specimen has suffered compression, causing it 

 to appear more elongate than originally. This distortion probably 

 affected the posterior half more than the anterior. It is believed 

 that the normal form should be higher, possibly intermediate between 

 what it is now and the restored figure of T^. scitula on the same plate. 

 Bearing this in mind, the species may be briefl}^ characterized as 

 follows: Carapace rather elongate, oblong to subrhomboidal, the 

 height four-ninths to possibly three-fifths of the length; ends subequal, 

 hinge straight, cardinal angles not very sharply defined, ventral margin 

 gently convex. Surface strongly convex, the point of greatest promi- 

 nence beneath and considerably in front of the middle; tubercles 

 quite obsolete. Surface of shell with the usual punctate ornament; 

 color yellowish white. 



Dimensions of the specimen described: Greatest length, 5.5 mm., 

 greatest height, 3.0 mm.; thickness of one valve about 1.3 mm. 



Remarks. — The unique example on which this species is founded 

 has precisely the same kind of shell as all of the other species referred 

 to the genus. It can not, however, be claimed as a typical member 

 of the genus, the entire obsolescence of the median tubercle and the 

 unusual length being such obvious departures from the average forms 

 of Walcottella, that its classification is necessarily attended with some 

 doubt. Still m the matter of elongation, the present species barely 

 exceeds W. oblonga, which doubtless is its closest known relative, 



