126 PALEONTOLOGY OF NEW-YORK. 
168. 15. ORTHIS SUBQUADRATA (n. sp.). 
Pu. XXXII. A. Figs. 1 a-o. 
Subquadrate, the cardinal line forming one side, the sides and base being nearly straight, 
with the angles rounded ; cardinal line less than the width of the shell, extremities curved ; 
area small, partially common to both valves; foramen moderate, triangular, partially 
common to both valves; dorsal valve nearly flat or slightly depressed near the margin, 
elevated towards the beak, which is small and well defined ; ventral valve regularly convex, 
with a shallow sinus along the centre, producing a slight elevation of the dorsal valve in 
front; surface marked by uniform subangular radu, which bifurcate near the beak, and 
again towards the margin, those near the cardinal line curving upwards ; radii crossed by 
fine elevated concentric lines, which are very distinct in the depressions between the rays. 
This species and the three succeeding ones are often confounded ; but if the following 
points are observed, there will be little difficulty in distinguishing the one under considera- 
tion by the external characters, the internal markings being quite distinct. 
The form of a perfect specimen is subquadrate, as shown in the figures ; the surface is 
evenly marked by radii which bifurcate twice, very rarely three times, before reaching the 
margin : the upper lateral ones curve upwards, so as often to run out upon the cardinal 
line, or the rounded slope near its extremity. This character is never observed in either of 
the following species. The visceral impression in the dorsal valve is subquadrangular, and 
broad below, the base being nearly on a straight line. The foramen of the ventral or convex 
valve is partially filled by a thick medial tooth, which projects above the plane of the area, 
and is deeply striated on the two sloping upper and outer sides. 
By comparing the parts indicated with either of the two following species, there will be 
no difficulty in deciding as to which the individual belongs. 
1 a, b,c. Three views of the dorsal valve, showing gradation in the size of the shell. 
1 d, e. Profile views, showing the nearly equal elevation of the beaks. 
1 f, g. Front views, showing the slight elevation produced by the sinus in the ventral valve. 
1h, 7. Ventral valves, showing the slight depression along the centre of the shell. 
. 1k. Cardinal view, showing the area, foramen, etc. 
ig 17. Exterior of a large dorsal valve somewhat flattened from compression. 
Fig. 1 m. Interior of the same, showing the form of the visceral impression and the marks of the external 
radii, which terminate near the margin. 
Fig. 1 2. Dorsal area and foramen. 
Fig. 1 0. Interior of the ventral valve, showing the narrow area and foramen, with the projecting medial 
tooth, which is enlarged in the upper figure. The impressions of the external radii are 
visible but a short distance from the margin on the inside of the shell. 
Position and locality. This is a common species in nearly all the western localities, being 
known at Maysville (Kentucky), Cincinnati and Oxford (Ohio) , Madison (Indiana), and 
other places, associated with other species of the genus, and with Leprmna. 
