PELECYPODA—PRIONODESMACEA. 



455 



251. M. subquadrata Shumard. (Fig. 603.) Carbonic-Permic. 

 Large. Left valve more convex than right which is nearly flat 



[Doctor Hind suggests that this is the 

 equivalent of the English Naiadites qnad- 

 rula (Sowerby).] 



Carbonic : Colorado, Arizona ? Also in 

 Permic (Matfield) of Kansas. 



252. M. perattenuata Meek and Hayden. 

 (Fig. 604.) Carbonic and Permic. 

 Hinge line very oblique to body of 



shell. 



Carbonic : Illinois-Colorado. Permic : 

 Kansas, Oklahoma, Texas. 



253. M. aviculoides Meek and Hayden. 



(Fig. 605, a.) Permic. 



Beaks slender, greatly prolonged, with a 

 slight upbending at the point. Umbonal ridge placed near an- 

 terior border and parallel with it, angular and very convex, giving 

 the shell an almost vertical anterior face. 



Kansas, Oklahoma, Texas, Utah ? 



254. M. permiana Swallow. (Fig. 605, b.) Permic. 

 Hinge line shorter than width of shell below. Differs from M. 



FlG. 603. Myalina sub- 

 quadrata, right valve, X Yz- 

 (Ind. Survey. ) 



Fig. 604. Myalina perattenuata. (Kan. Pal., VI/II. 

 Fig. 605. a, Myalina aviculoides, right valve ; b, M. permiana, left valve. 

 (After White.) 



aviculoides in its less convex and narrower valves, more rounded 

 umbonal ridge, and in wanting the upturned beaks. 

 Kansas, Oklahoma, Texas, Utah ? 



LXXVI. Ptychodesma Hall and Whitfield. 

 Anterior end short, posterior end broadly rounded. Hinge line 

 short. Surface concentrically striated. Hinge with two or more 



