206 PALEONTOLOGY OF NEW YORK. 



faunas of tlie Lower Helderberg and Oriskany groups. Eatonia bears very 

 much the same relation to Uncinulus as the subgenus Pugnax to the cuboidal 

 shells of Hypothyris. The species of Eatonia possess two quite distinct types 

 of exterior, one strongly plicated over fold, sinus and lateral slopes, the other 

 radially lineate, with broad margins of contact, which are usually crenulated as 

 if by the extremities of the rounded plications. To the former belong E. medi- 

 alis, Vanuxem, and E. eminens, Hall, of the Lower Helderberg group, E. sinuata 

 and E. Whitfiddi, Hall, of the Oriskany sandstone ; to the latter, E. singularis, 

 Vanuxem, of the Lower Helderberg, E. peculiaris, Conrad, of that fauna and of 

 the Oriskany sandstone, and E. pumila, Hall, also of the Oriskany sandstone. 



Gends CYCLORHINA, gen. nov. 



PLATE LXI. 



1860. Rhyndwspira, Hall. Thirteenth Rept. N. Y. State Cab. Nat. Hist., p. 83. 



1867. Trematospira?, Hall. PalEeontology of New York, vol. iv, p. 412, pi. Ixiii, figs. 33-36. 



1889. Retzia {Trematospira), Whiteaves. Contrib. Canadian Palaont., vol. i, pt. 2, p. 116. 



Shells of comparatively large size at maturity, subtriangular in outline ; 

 biconvex, the convexity of the brachial valve being the greater. Fold and 

 sinus very broad, and developed in the usual manner, on brachial and pedicle- 

 valves respectively. 



On the pedicle-valve the apex is obtuse, not elevated, and is very broadly 

 truncated by a large circular foramen, which, even in the earliest growth-stages 

 observed, is enclosed for fully five-sixths of its periphery by the substance of 

 the valve. The deltidial plates are incipient at maturity and scarcely evident 

 in young shells ; the delthyrial margins are extremely divergent. The cardinal 

 line is short but straight, and its extremities are produced on each side to form 

 a short alate process or wing, similar to those in the genus Eumetria. These 

 extensions occur on both valves, and are very apparent in the younger shells, 

 but become somewhat obscured with the increase of convexity accompanying- 

 maturity. On the interior, the teeth are large and blunt, and attached to the 

 lateral walls of the shell, though they also rest upon the thick lamellae similarly 

 attached except at their anterior margins, and which converge downward to 



