414 A. HYATT — TRIAS AND JURA IN THE WESTERN STATES. 



PELECYPODA. 



Monotis semiplicata, n. s. 

 5^^\V Loc, American canyon. 



This shell has a more elongated form than Monotis subcircluaris, 

 being narrow anteriorly and then widening out posteriorly. There are 

 concentric lines over the whole valve, but only the umbonal ridges and 

 posterior parts of the valves are adorned by radiating ridges. These 

 consist of coarse, linear, straight ridges or costse widely separated and 

 with finer lines between them. The extreme border of the posterior part 

 of the shell in the vicinity of the hinge is also destitute of radial lines. 

 There is a distinct but very small posterior wing, and the anterior edge 

 has also a very slight extension, just enough to make it appear straight. 

 The anterior or oral region is short, the umbo varying from nearly ter- 

 minal to about one-fifth of the antero-posterior diameter from the oral 

 end. The posterior end is much broader, the shape being an extended 

 oval, narrower anteriorly than posteriorly. The umbo is situated nearer 

 the center in young specimens, but always well toward the anterior end^ 

 and the shape of the valve does not change materially with growth. The 

 species is gregarious, thickly crowded on the slabs, having had similar 

 habits in this respect with Monotis sallnaria and sub circular is. It is obvi- 

 ously either a Triassic or Rhetic species. 



Monotis symmetrica, n. s. 

 ' Loc, American canyon. 



The shells of this species, although found at the same locality in the 

 canyon, differ • from those of M. semiplicatus most markedly in outline 

 and the position of the umbones. These are almost central in young 

 shells, and about one-third of the antero-posterior diameter distant from 

 the anterior oral edge in adults. The shape is irregularly elliptical, the 

 long hinge line forming the upper, flattened part of the outline. The 

 anterior end is narrower than the posterior and the branchial edges 

 evenly rounded and gibbous. There are regular concentric ridges and 

 some radial folds along the umbonal ridges much slighter than in M, 

 semiplicata and apparently absent in one adult and one young speci- 

 men. It is much rarer than its companion species, only seven speci- 

 mens having been found on the slabs with Monotis semiplicatus. The 

 aspect is that of a species of Daonella, but the evident affinity with 

 Monotis semiplicata is vouched for by the presence of similar posterior 

 and slight anterior wings. 



