SILURIAN FAUNAS. 235 
back of the centre of the valve. Brachial valve most prominent at 
and in front of the middle; mesial fold not conspicuous, except near 
the front margin. Each valve marked by from eighteen to twenty-two 
simple, angular plications, three of which are usually included in, 
the sinus of the pedicle valve. The finer markings of the shell, if 
they were present, have been obliterated by exfoliation. bat 
The dimensions of an average adult specimen are: length, 9 mm.; 
width, 9.5 mm., and thickness, 5.5 mm. 
Remarks.—This little shell is one of the most abundant species in 
the Decker Ferry formation, being present in each of the faunal zones. 
It is especially abundant in the lower beds. The species resembles. 
R. transversa of the Helderbergian fauna, and may be ancestral to if, 
but the mesial fold of the brachial valve is never so much elevated 
in front as in the adult individuals of that species. The species also 
resembles Camarotechia neglecta Hall, as illustrated in volume [T. 
of the New York Paleontology, from the Clinton and Niagaran faunas, 
but it does not at all resemble the shell from the Niagaran fauna at 
Waldron, Indiana, to which that name is applied. 
WILSONIA GLOBOSA DN. Sp. 
Plate XXI., Figs.. 12-22. 
Description.—Shell subglobose, a little longer than wide. Pedicle 
valve less convex than the brachial, its beak suberect or slightly arched,. 
acutely pointed, umbo smooth and convex, mesial sinus shallow, be- 
ginning near the middle of the valve and produced as a lingual ex- 
tension in front, at nearly a right angle to the plane of the valve. 
Brachial valve strongly convex or gibbous, smooth posteriorly, anterior 
margin deeply sinuate, mesial fold slightly elevated, originating near 
the middle of the valve. The surface of each valve is marked by 
sixteen or eighteen simple, low, rounded plications, sometimes slightly 
grooved anteriorly and becoming nearly or quite obsolete posteriorly, 
leaving that portion of both valves smooth. From two to five plica- 
tions are included within the sinus, the more common number being 
four, with a corresponding number in the fold of the opposite valve. 
The dimensions of a rather large globose individual are: length,: 
12 mm.; width, 11 mm.; thickness, 10 mm. Those of another less: 
globose specimen are: length, 11.25 mm.; width, 10.5; thickness, 
7.5 mm. 
