32 PROCEEDINGS OF THE ACADEMY OF [Part I 



the beak sculpture when known is of the concentric type.^^ These 

 were probably immigrants from Asia. Similar forms occur more 

 abundantly in the Laramie. 



Williston, from a study of the reptiles, holds that America was 

 isolated from the Old World throughout the Permian and well 

 into the Triassic, a broad connection being established in the Upper 

 Trias.2'' It appears likely that the subfamihes of Unionidse were 

 differentiated in this long interval from the Pennsylvanian to the 

 Trias, the Unioninae in Eurasia, while America had Hyi'iine mussels, 

 which disappeared with the advent of Old World forms in Upper 

 Trias or Jurassic. The migration of Unionidse would doubtless 

 lag far behind that of reptiles after the connection was established. 



Diplodon pennsylvanicus n. sp. PI. II, figs. 1 (type), 2, 3; pi. Ill, fig. 4. 



The type consists of the two valves of one individual, spread 

 open. On the same piece of hard gray shale there is another imper- 

 fect valve of the same species. 



The shell is oblong, not unlike a young Ufiio complanatus (Sol.) 

 in shape, being rather compressed. The beaks are at about the 

 anterior third. The posterior ridge is rounded, a radial depression 

 above it on the posterior slope. Sculpture of numerous folds radi- 

 ating from the beaks and reaching to about the middle of the valve 

 except posteriorly, where they are longer. Below the radial folds 

 there are some low, concentric wrinkles. 



Length 21 mm.; alt. 10.5 mm.; diam. about 6 mm. 



Locality: Shale, Little Conewago Creek (loc. 5). 



The specimen selected as type is not full grown, but the valves 

 are free from distortion, and the sculpture is finely preserved. Adult 

 shells, such as that shown in Plate III, figure 4, reach a length of 32 

 mm., alt. about 18 mm., diam. about 10 mm. The radial sculpture 

 covers a relatively smaller part of the valves, though actually about 

 the same area as in the type. 



Another specimen, obliquely compressed, is shown in dorsal aspect 

 in Plate II, fig. 2. Alt. about 20 mm. 



-^ In some of the Laramie and later Uniones there are obHque folds, super- 

 ficially like those of Diplodon, as in Unio {Loxo pie urns) belliplicatus Meek; 

 but on the beaks the sculpture is concentric. Certain Uniones described by 

 Whitfield, Bull. Amer. Mus. N. H., XIX and XXIII, from the Montana Laramie, 

 have sculpture recalling Diplodon, but more like the Asiatic Parreysia. They 

 form, I think, a special group, near Parreysia or a subgenus thereof, which may 

 be called Proparreysia, the tyi)e being Unio percorrugata Whitfield. 



" This connection is usually mapped as across the North Atlantic (see Arldt, 

 Handbuch der Palaeogeographie, I, 1919); but it may have been in the north- 

 west. 



