BEEDE: UPPER PERMIAN RED BEDS. 149 



indication of the fold and sinus first appears in pretty large 

 individuals and shows the ordinary fold of a Dielasma, the 

 biplicate character appearing only in well-developed indi- 

 viduals. Along with this development comes a peculiar 

 change in the form of the shell. It nearly ceases to grow 

 laterally, and the additional growth takes place largely on 

 the anterior margin, which narrows and becomes cuneate in 

 longitudinal section. At the time when the biplication first 

 becomes noticeable the shell is relatively very much broader 

 than in the adult stage. The dental lamellae are well devel- 

 oped in the adult individuals. They began to appear at a 

 somewhat earlier stage than the biplication, but are not 

 shown until the specimens reach a length of over seven milli- 

 meters. These plates, in the cast, give the appearance of a 

 subquadrangular foramen ; however, the foramen was nearly 

 circular. In the brachial valve the features are similar to 

 those of the pedicle valve, except that it is more convex and 

 its lateral edges much more elevated. In the young speci- 

 mens the muscular impressions seem to have sunken deeply 

 into the shell itself. In larger ones the shell seems nearly 

 plain within, while in the adult specimens there seems to be 

 a cast of a true platform characteristic of the Diela.'unas. 

 The crural lamellae are not very strikingly developed, though 

 present. No specimens with the loop preserved have been 

 seen in the fossils from Oklahoma. Two specimens from 

 Texas show the loop very well, as shown on plate V. One 

 of these is somewhat smaller than the other, and the loop is not 

 yet united ; in the larger one it is complete. These speci- 

 mens are not over half grown. The adult form would doubt- 

 less show some further modifications. 



Whitehorse spring, Oklahoma ; very abundant. Dozier, 

 Tex. ; very abundant. 



There is a large valve in the Oklahoma collection showing 

 four plications instead of two. This is probably a pathologic 

 specimen, or may represent another species. The tendency 

 to produce abnormalities in this fauna is rather marked. 



The surface of specimens of this species seems to have been 

 smooth except for occasional growth marks. The shell was 

 punctate, as is clearly shown in a specimen with a portion of 



