BRAOHIOPODa. 763 
Cyrtina, Davidson. 1858. 
(Plate 29, figs. 12-26.) 
Shells usually of small size and semi-pyramidal contour. Pedi- 
-cle-valve with a high, vertical or arched cardinal area, which may 
be unsymmetrical from distortion or unequal lateral growth ; this 
area is divided medially by an elongate-convex deltidium, which 
may be perforated at any point below the apex, by a circular, 
direct or oblique foramen, or be without any evidence of such 
foramen. When present the foramen is accompanied by a sinus 
on the deltidium, extending from it to the apex of the valve; 
even when this foramen has been closed from senile deposition of 
testaceous matter this foraminal groove may remain. 
The exterior surface bears a median sinus and more or less dis- 
tinct lateral plications. On the interior the dental lamelle are 
strongly developed and converge rapidly, meeting a median sep- 
tum from the bottom of the valve. The union consists of a lat- 
eral junction of the dental lamellz with the septum, the latter 
continuing for a short distance beyond the point of confluence as 
a vertical ridge, always apparent in the bottom of the spon- 
dylium thus formed. At the point of union these three plates 
constitute a tubular chamber which has no external opening in 
older shells, and may be filled by organic deposit. The dental 
plates are shorter than the septum, the latter, at its base, extend- 
ing beyond the center of the valve, its anterior margin being con- 
cave and its inner extremity acute and produced. 
Brachial valve very shallow, with narrow, inconspicuous car- 
dinal area. Surface plicated as in the opposite valve. Cardinal 
process consisting of a double apophysis on the sides of which 
are strong, divergent crural plates. The spiral cones are elon- 
gate-fusiform, each coil attaining its greatest diameter just below 
the center. They are directed obliquely upward and backward 
toward the middle of each lateral slope of the pedicle-valve. 
The jugum is continuous, its branches being directed upward and 
forward, uniting at their extremities. The muscular impressions 
comprise two oval anterior, and fainter posterior scars. The sur- 
face ornamentation consists of radial plications which may cover 
both fold and sinus; in rare instances the lateral plications are 
absent. The concentric growth-lines are sometimes fine and 
15 
