PLATE 21. 
Page. 
BIG OSTR BAN TNS CU RTS 52) ee eS Se Ve eee ce ag a a es a 56 
a. Lower valve ; exterior view; natural size. 
b. Interior view of the same. 
Fic. 2. CORBICULA (LEPTESTHES) PLANUMBONA.-..-..----.----- see diahate oa 4 rue 
a. Right side view of an adult example; natural size. 
b. Dorsal view of the same. 
e. Right side view of another example. 
d. Lett side view of a large example. 
HIG. 3. CORBICULA OCCIDENTALIS... 2.005.425 oases ones aes ine che aie anu ea ris) 
a. Right side view of an example from Yampa Valley, Northwesteru 
Colorado; natural size. 
b. Right side view of Mr. Meek’s type of C. bannisteri. 
c. Dorsal view of the same. 
Fig. 4. CORBICULA CYTHERIFORMIS ......---.-- PAE AVES Guan eve Red NIUE RO AE 74 
a. Right side view ; *natural size. 
b. Dorsal view of the same example. 
e. Interier view of a right valve. 
d. Interior view of a left valve. 
. CORBICULA (LEPTESTHES) FRACTA ......-- PREM Seagate neces RON US AAs cas ys 75 
a. Left side view of an imperfect example, showing an unusually 
elongate form. 2! 
For other examples see plate 23. 
Fig. 
On 
Fig. 6. NERITINA VOLVILINEATA......--------.--- Psy ANOS Pamir cantl DIGI CaS A LS 2 8 88 
a. Lateral view ; enlarged to ene and a half diameters. 
b. Apertural view of the same. 
