AET. * CAMBRIAN CONCHOSTilACA — ULEICH AND BASSLER 87 



observed at the posterior edge, and at least less evident on the ventral 

 side. Gape of valves narrow, nearly uniform in width in the anterior 

 and ventral parts, but expanding slightly in the posterior. Most of 

 gape made up by the right valve. Test thin, black to light gray, 

 usually exfoliated, the outer surface rarely seen, distinctily punctate 

 when preserved. 



Dimensions of rather large example: Greatest length, 10.0 mm.; 

 greatest height, 6.2 mm.; greatest thickness of single valve, about 

 2.0 mm.; of entire carapace, about 4.2 mm. 



By far the most of the specimens studied retain both valves, 

 which, considering the coarseness of the sandy matrix, probably 

 indicates a strong hingement of the valves. As the hinge is not 

 denticulated, and as the valves are not kept in place by overlap of 

 their edges, it is assumed that comparatively strong muscles, probably 

 located in the umbonal part, were responsible for their retention in 

 place. The small depressions in the casts of the antero-dorsal 

 angle are thought to indicate the lodgment of these muscles. 



Occurrence. — Common in the Bright Angel shale, head of Nunko- 

 weap Valley, Grand Canyon of the Colorado, Ariz. D. nasuia and 

 D. appressa are comparatively rare at the same locality, but species 

 of Walcottella were not observed. 



Cotypes.—Cht. No. 56510, U.S.N.M. 



DIELYMELLA RECTICARDINALIS ANGUSTATA, new variety 



Plate 10, Figure 8 



The interior cast of an entire carapace figured on Plate 10 is all 

 that has been seen of this variety. It is considerably smaller than 

 the average for specimens of the typical form of the species. That 

 it is not merely a young stage of the typical variety is demonstrated 

 by the figure of a smaller individual of the latter beside it on the same 

 plate. (Fig. 7.) In the absence of other specimens, the writers 

 hesitate to separate the individual under discussion as a distinct 

 species, the subordinate designation above proposed being thought 

 provisionally sufficient to cover the case. 



On comparison the variety angustata is readily distinguished by its 

 more elongate form, the length (7.0 mm.) being fully twice the height 

 (3.5 mm.) whereas in the typical variety the height scarcely exceeds 

 two-thirds of the length. In the former the antero ventral part of the 

 outline is less prominent, the recession of the curve beginning nearer 

 the dorsal angle. The posterior end also is somewhat different, the 

 point at which the outline bends forward being opposite the mid- 

 height^ instead of above it. All of these peculiarities might have been 

 produced by accidental compression of the ventral half of the cara- 

 pace; but as the specimen exhibits no evidence of such distortion^ 

 indeed, as none of the fossils from this bed seem to have suffered 



