BRAOHIOPODA. 815 
and slender, and constitute the crural bases. Spirals are present, 
but their direction and the nature of the loop are undetermined. 
A stout median ridge supports the hinge-plate and divides the 
scars of the adductor muscles. In both valves the lateral por- 
tion of the umbonal region is pitted. The plications of the sur- 
face are covered by fine, sharp and elevated concentric striz. 
Shell substance, fibrous, impunctate. 
Type, Clintonella vagabunda, Hall. 
Distribution. Upper Silurian (Clinton group). 
Atrypina, Hall. 1893. 
(Plate 41, figs. 1-6.) 
Shell small, subovate or subcircular in marginal outline, plano-, 
or subconcavo-convex in contour; surface coarsely and sparsely 
plicated . 
Pedicle-valve with the umbo prominent, the beak abruptly 
acute and more or less incurved. Foramen apical, and deltidial 
plates normally developed. The cardinal margins of the valve 
are somewhat extended in the typical species, though the hinge 
itself is quite short. Teeth divergent and unsupported, taking 
their origin on the lateral cardinal slopes, and very slightly 
recurved. Muscular scars exceedingly faint; no internal septa 
observable. 
Brachial valve with the cardinal process small, consisting of 
two short lobes, which meet at their apices, not extending back 
of the hinge-line, and diverging anteriorly. The surface of each 
lobe may be longitudinally grooved, but the inner and outer 
divisions thus formed are confluent at 
their outer extremities. The anterior face 
of the process is abrupt and vertical, its 
lower portion being continuous with the 
socket walls. In front of the cardinal pro- 
cess, but not supporting it, is a low median 
ridge, on either side of which are obscure 
muscular imprints. The brachial appar- 
atus consists of introverted spirals whose 
Ef B Fia. 398.— The brachidium of 
bases lie against the lateral slopes of the 4trypina disparilis, Hall. 
pedicle-valve and whose apices are directed toward the center 
of the brachial valve. The ribbon is loosely coiled and makes 
67 
