66 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



therefore, their occurrence does not extend the known range of the 

 genus. 



CAMERISMA PRAVA Grant n. sp. 



Plate 8, figs. 1-1 b; figure 11 



Shell about average size for genus, strongly biconvex ; outline trans- 

 versely subelliptical or rhomboid, typically asymmetrical, distorted; 

 commissure strongly uniplicate at anterior ; fold high, sharp-crested, in 

 form of Gothic arch, crest curving left or right toward anterior ; sulcus 

 moderately deep, median trough sharp, curving left or right to match 

 fold ; costae entirely absent ; posterolateral edge of pedicle valve with 

 narrow flange overlapped by edge of brachial valve; edges of valve 

 elsewhere meeting at obtuse or right angle, without protruding, with- 

 out trace of stolidium; concentric growth lines fine, closely spaced; 

 growth laminae only slightly stronger, strongest and most frequent 

 near margins. 



Pedicle valve strongly convex longitudinally through sulcus ; um- 

 bonal region slightly swollen; beak short, thick, erect to recurved 

 against brachial umbo, therefore foramen not observed. 



Brachial valve flatly convex longitudinally along crest of fold, 

 strongly convex transversely ; beak bluntly pointed, apex hidden within 

 pedicle valve. 



Pedicle valve interior with dental plates forming spondylium ele- 

 vated on low median septum, not sessile in apex; septum becoming 

 gradually higher anteriorly, remaining rather low, extending anteriorly 

 about one-third length of valve. 



Brachial valve interior with short, undivided hinge plate in poste- 

 rior, apex of plate slightly swollen to form cardinal knob for attach- 

 ment of diductor muscles ; camarophorium beginning in apex as thick- 

 ening of crest of median septum, becoming definitely spoon-shaped 

 anteriorly; height of septum increasing anteriorly, curving camaro- 

 phorium ventrally ; strong intercamarophorial plate connecting midline 

 of camarophorium to underside of hinge plate ; crura extending ante- 

 riorly from hinge plate, curving ventrally like camarophorium. 



Holotype.—USm/L 172074, plate 8, fig. 1. 



Comparisons. — Camerisma prava is characterized by its wide outline, 



Table 10. — Measurements of Camerisma prava from the Mississippian of Alaska 



Measurements, in millimeters 

 USNM No. Length 



142072 15.0 



142073 15.3 



142074 c.17.0 



