I12 Cincinnati Society of Natural History. 
It should be mentioned also that in some examples the posterior 
lobe appears to be a little flattened on top, and the ventral prolon- 
gation of the anterior lobe forms occasionally a more pronounced 
ridge than shown in fig. 5a. 
Position and locality: Middle beds of the Cincinnati Group, at 
McKinney’s Station, Lincoln County, Ky. 
TETRADELLA, Nn. gen. 
Syn. Leyrichia, of authors (in part); Strepula, Ulrich (non Jones 
and Holl.) 
This genus as now constituted comprises two distinguishable 
groups of species. ‘The first and typical section may be defined 
as follows: 
Carapaces somewhat oblong, often subquadrate, never tumid, with 
the hinge line straight. Surface depressed, with a semi-circular, 
narrow ridge rising abruptly from the free margins and running 
close to and nearly parallel with them, terminating at the poste- 
rior and anterior dorsal angles. In the enclosed space, two often 
slightly modified narrow ridges traverse the valves in a vertical or 
oblique direction from the dorsal edge, or from points near it, to 
the posterior half of the ventral portion of the sub-marginal ridge, 
uniting with it. The four vertical ridges of the dorsal half of the 
valves have suggested the name, but the fact that they all unite 
below is perhaps the most significant character of the genus. 
Types: Beyrichia quadrilirata, Hall and Whitfield, Pal. Ohio, 
Vol. Il, p. 105, Plate IV, figs. 6 and 7, 1875,*% and Z. subguad- 
vans, N. Sp. 
Other species that should be placed here are Strepula quadrilirata, 
var. somplex, Ulrich, described from Manitoba, which proves to 
be a constant form, having lately been observed in considerable 
abundance on slabs from the upper beds of the Cincinnati Group, 
at Weisburg, Ind.; Strepula lunatifera, Ulr., and the following 
European species: Seyrichia complicata, Salter; 8. bussacensis, 
Jones; B. ribetriana, J., and B. affinis, J. 
The second section, of which Beyrichia eculifera, Hall, and BL. 
chamberst, S. A. Miller, are to be regarded as the types, differs 
* More correctly figured and described in Contri. to the Micro-Pal. of the 
Cambro-Sil. Rocks of Canada, Pt. IJ, p. 54, Pl. 1X, fic) 12 goaoeeeue 
author here erroneously refers the species to Stregula, Jones and Holl. 
