728 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
SOWERBYITES LAMELLOSUS Cooper, new species 
Plate 180, C, figures 17-25; plate 181, B, figures 10-18; plate 184, G, figures 23-27 
Shell of about usual size for the genus, length about two-thirds the width; 
hinge equal to or forming the greatest shell width; lateral margins nearly straight 
in the posterior half; anterolateral margins well rounded; anterior margin 
broadly rounded to truncated. Cardinal extremities varying from a right angle 
to acutely angular and subalate. Surface of exfoliated specimens costellate, with 
about 12 of the larger costellae occupying a space of 5 mm. at the anterolateral 
margins and 4 to 8 of the finest costellae occupying the spaces between the larger 
ones. When not exfoliated the shell is covered by extremely fine lamellae which 
are longest at the geniculated portion or anterior third of the pedicle valve. 
Lamellae of brachial valve not well developed. 
Pedicle valve varying from gently convex to moderately convex in lateral pro- 
file with the greatest convexity at about the anterior two-thirds. Umbonal region 
slightly inflated with the slopes to the cardinal extremities gentle; anterolateral 
slopes steep; anterior slope steep. Interarea gently apsacline to orthocline. 
Pseudodeltidium short. When the lamellae of the exterior are present, the steep- 
ness and narrow convexity of the anterior third are emphasized. 
Brachial valve gently concave in lateral profile with the greatest concavity lo- 
cated at the anterior third where the valve is somewhat sharply bent toward the 
brachial valve. Posterior two-thirds nearly flat but with a faint median sulcus 
originating at the beak and extending to the front margin in some specimens. 
Interarea short, hypercline. Cardinal extremities slightly deflected toward the 
pedicle valve. Exterior lamellae shorter and not so well developed as in the 
pedicle valve. 
Interior of pedicle valve with outside margin of muscle area forming a right 
angle. Adductor impressions deeply sunk and located within the delthyrial cham- 
ber. Adjustor-diductor scars straight with outer margins slightly rounded, ex- 
tending slightly anterior to the middle of the valve. Anterior callosities moder- 
ately thick. 
Interior of brachial valve with low, rounded, thick median septum and 2 low 
but wide lateral septa, the inner lateral septum being the stronger of the two. 
Cardinal process moderately elevated ; callus deposit about the cardinalia thick. 
Anterior slope steep ; anterior end of median septum only slightly elevated. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
PAGIOUV OE nic gticnn eRe ees adinve 13.8 12.8 18.9 19.7+ 4.9 
Paratype’ (TI74G3a) s'. caes tye 13.3 12.0 21.3 22.8 2.8 
m (LE7AOSA) SS. oe 13.4 12.7 22.9 23.9-++ 3.0 
Se (BIAAOSC) 04 i. BSS 12.5 11.6 2r5 212 28 
‘ CEIFAOGE a cha s stirs scase 13.1 12.8 21.6 22.7+ 3.9 
Types—Holotype: 117406c; figured paratypes: 117402a-c,h, 117403a-c, 
117405a-d ; unfigured paratypes: 117402d-g, 117406a,b,d,e. 
Horizon and locality—Bromide formation (Mountain Lake member) in Okla- 
