152 



PALEONTOLOGY OF NEW YORK. 



Gbnus glass I a, Davidson. 1881. 



1849. Atrypa, Sowbrby. Silurian System, pi. viii, fig. 9. 

 1859. AtMyrUf Saltbr. Siluria, second ed., p. 642, pi. xxii, fig. 16. 



1867. Athyris, Davidson. British Silurian Brachiopoda, p. 121, pi. xii, fig. 19; pi. xiii, figs. 5, o. 

 18SI . Olastia, Davidsok. Geological Magazine, new series, vol. viii. 



1882. Olastia, Davidsoh. British Devonian and Silurian Brachiopoda Supplement, p. 38, pi. i, figs. 

 10-14; pp. 116-120, pi. vii, figs. 9-20. 



Shells small, biconvex; elongate-ovate in outline; surface smooth. Umbo 

 of the pedicle-valve not conspicuous; beak depressed. Structure of the 

 deltidium and hinge as in Nucleospira. Muscular impression consisting of 

 two widely divergent, oval diductor scars, between which lies a broad ad- 

 ductor scar. 



i . :i:. Fio. 143. 



Figs. 142-141. OUu$ta obovata, Sotrerby. 

 Fig. 142. Interior of the pedicle-valve. 

 Figs. 143, 144. Views uf the exterior. Natural size. 



FIO. 144. 



(Davidson.) 



Brachial valve with an internal septum. The spiral cones have their bases 

 toward the lateral margins of the shell and their apices at the center of the 

 internal cavity ; their position with reference to each other is therefore just 

 the reverse of that in Meristella, Retzia, etc. The cones are laterally com- 

 pressed, and the ribbon makes but few volutions. The loop originates as 

 in Atrypa, is continuous, bending downward into the space between the 

 cones and making a sharp angle at the point of union, which may be directed 

 upward. 



Type, Atrypa obovata, Sowerby. Wenlock and Ludlow formations. 



In this genus and Cyclospira the spirals are at the extreme of introversion, 

 and the structure of the brachidium in its entirety is quite similar to that 



