Silurian Fossils 
91 
I the other radiating plications in size anteriorly, and are but slightly 
I depressed below the level of radiating plications on either side. 
I About 6 distinct radiating plications on each side of the median 
i groove. 
I Brownsport bed: Bath Springs, Tennessee. 
I A study of the various specimens of Homceospira at hand sug- 
gests that there is considerable variation in the appearance of the 
plications which occupy the median groove even within the limits 
of the same species so that characters drawn from this source are 
considered of doubtful value. A single plication may begin at 
different distances anterior to the beak and may or may not bifur- 
cate anteriorly; in case it bifurcates the two branches may be very 
fine and remain very close together, or they may separate more 
or less; they may separate so far as to occupy a position along the 
side walls of the groove. In some cases these median plications 
may so nearly equal the size of the plications on either side that 
only their later insertion at a greater distance from the beak dis- 
tinguishes them readily. Sometimes two plications are inserted 
side by side along the median groove and increase in size anteriorly 
or remain comparatively small. Nevertheless differences in form 
are noticed which are sufficiently constant to suggest the presence 
of different species. These differences consist chiefly in differ- 
ences in the lateral outline, the convexity, and the coarseness and 
, distinctness of the plications. The amount of overarching of the 
I beak of the pedicel valve varies considerably in the same species. 
Cyrtia cliftonensis. 
I {Plate II, Fig. S2.) 
I Height of cardinal area, 6 mm.; width, 10 mm.; margins of sinus 
I of ventral valve distinct and angular, diverging at an angle of about 
52°; lateral outlines of the cardinal area form an angle of about 
' 82° at the beak; the cardinal area forms an angle of about 69° with 
a plane resting upon the margins of the sinus. Although the sinus 
is sharply defined laterally, it is of only moderate depth; in a sim- 
ilar manner the elevation of the median fold on the dorsal valve 
is moderate. Surface with very fine radiating striae, seen only 
under a lens. Width of delthyrium at hinge line about i mm.; 
I foraminal groove of the deltidial covering apparently very long 
I and narrow but not well preserved. 
