44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
Measurements in mm.—Holotype, surface length 18, length 11.9, 
hinge width 17.9, midwidth 15.2, height 7.6; thickness of a com- 
plete specimen (U.S.N.M. 125364b) near middle 2.8. 
Types.—Holotype, U.S.N.M. 125364a; unfigured paratype, U.S. 
N.M. 125364b. | 
Localities —L8; S12, S40, S41, S49, S56; U.O. 2204-3. 
Discussion—This species is characterized by its deep and wide 
sulcus. It differs from all others described herein by this feature, 
but its interior is unknown and the generic assignment therefore is 
uncertain. 
ALEXENIA? OCCIDENTALIS Cooper, new species 
Plate 2D, figures 25-34 
Shell of about medium size, slightly wider than long but with a 
long trail. Greatest width at middle, hinge narrow, ears strong, 
obtuse, flattened but not extended. Margins strongly rounded; pos- 
terior third moderately coarsely reticulate. Anterior two-thirds cos- 
tellate with costellae narrowly rounded and separated by spaces 
nearly as wide as the costellae. Costellae numbering about 5 to the 
millimeter on the trail. 
Lateral profile of pedicle valve strongly curved. Ears broken. 
Umbo narrowly but strongly swollen, with steep slopes; median 
region inflated. Sulcus originating 10 to 15 mm. anterior to the beak, 
shallow and broadly U-shaped, disappearing or becoming faintly 
defined anteriorly on the trail. Flanks bounding sulcus broadly in- 
flated ; lateral slopes rounded. Location of spines not clearly defined 
but spine bases are scattered over trail and lateral slopes. 
Brachial valve gently concave, most so in the umbonal region which 
forms a shallow pit and in anterolateral extremities. Median region 
defined by a low, narrow fold originating just posterior to the middle 
of the visceral disk region but becoming poorly defined or obsolete 
anteriorly. Cardinal process moderately long, slender, with bilobed 
ventral face. 
Measurements in mm.—Holotype, length 28.3, width at middle 
32.7, surface length 54, height 14.5; paratype, U.S.N.M. 125334 
(brachial valve) length 23.2, width 31.4, surface length 34, height 10. 
Types.—Holotype, U.S.N.M. 125350; figured paratypes, U.S.N.M. 
125334, 125335. 
Localities—614, 638, 639; F; Pro, P12; S12, S30, S39, S41, S49, 
S50, S103; U.O. 2201-4, 2202-2, 2204-3, 2211-2. 
Discussion—This species is characterized by the finely reticulate 
