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 plication. 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 



T\a. 137. FIG. 138. 



The brachidlam of Atrypa {Protoxyga) exigua. Hall. (o.) 



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 oflf, 

 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 



* Catalogaes of the Silurian Fossils of the Island of Anticosti, p. 47. 1866. 



t British Silurian Brachiopoda, Suppl., p. 121. 1862. 



t Paleontology of New York, vol. i, p. 141, pi. xixiii, figs. 6a-d. 1847. 



