BRACHIOPODA. 
149 
from Dayia, which, as already suggested, has only an external resemblance to 
this species. 
It is not known with certainty whether other representatives of this type of 
structure exist among the species of the earlier faunas. Mr. E. Billings de¬ 
scribed a species, Athyris Lara, from his Division 2 of the Anticosti group,* 
which has somewhat the form of Cyclospira bisulcata, and Mr. Davidson statesf 
that it contains introverted spirals. 
In another species, or series of species, we find abundant and convincing 
evidence of the existence of a slight modification of this type contemporaneous 
with Cyclospira bisulcata. The Atrypa exigua, Hall,:|; a diminutive shell described 
from the Trenton limestone of New York, has a similar contour to C. bisulcata, 
though the pedicle-valve is less convex and the ante-lateral margins of the 
valves bear evidence of coarse plic ation. In this little shell the brachial valve 
has a simply divided hinge-plate, and upon these divisions rest the two short 
convergent crura; joining the latter at a low angle, the primary lamellae diverge 
laterally, converge slightly toward their anterior margins, thence curve verti¬ 
cally upward, nearly touching the inner surface of the pedicle-valve and very 
gradually approaching each other. The ribbon is continued with a decided 
internal inclination, until it completes slightly more than one entire volution. 
Toward the anterior margins of the primary lamellae a strong loop is given off, 
its lateral branches projected very obliquely backward, sometimes scarcely 
rising between the coils, the union forming a broad angle on the anterior mar¬ 
gin with a subacute process on the outer margin. In the accompanying figures 
* Catalogues of the Silurian Fossils of the Island of Anticosti, ji. 47. 1866. 
t British Silurian Brachiopoda, Suppl., p. 121. 1862. 
J: Palseontology of New York, vol. i, p. 141, pi. xxxiii, figs. 6a-d. 1847. 
