766 Report oF THE STATE GEOLOGIST. 
not extend to the bottom of the valve. Between these arises a 
large median septum whichis horizontally expanded at its summit 
and toward its interior margin. In the brachial valve the dental 
sockets are deep, the hinge-plate very large, formed by the hori- 
zontal expansion and union of the crural plates and producing a plat 
Fics. 299-30!. — Suessia costata, Deslongchamps; showing the structure of the pedicle and 
brachial-valves. (SvEss.) 
form which extends to the middle of the valve. This plate bears 
the impressiom of the adductor muscles. The primary lamelle 
of the spiral coils are united by a jugum bearing a median process 
directed anteriorly. 
Type, Suessia costata, E. Deslongchamps. 
Distribution. Jurassic. 
Whitfieldella, Hall. 1893. 
(Plate 32, figs. 1-9.) 
Shells usually of small size; valves subequally convex, ovate 
or elongate in outline. Umbo of the pedicle-valve not high 
or greatly incurved, usually exposing the circular apical foramen, 
beneath which the deltidial plates are frequently retained. 
Cardinal slopes of both valves broad and not distinctly defined ; 
anterior margin subtruncate and gently sinuate. In the typical 
forms there is a faint sinus on both valves near the anterior 
margin, otherwise the surface is smooth. On the interior the 
muscular impressions of the pedicle-valve are similar to those 
of Meristetia. In the brachial valve the hinge-plate is concave, 
divided by a deep central concavity which is supported by a 
median septum. On either side are lobes bearing the bases of 
the crura. The brachidium consists of two spiral cones arranged 
18 
