LAMELLIBRANCHIATA. 573 
Whitella ventricosa,] 
angular or sharply rounded ana distinct quite to the postero-basal margin. An 
obscure furrow in the middle of the flat cardinal slope. From W. ventricosa Hall, sp., 
which seems to be its nearest congener, it differs principally in the greater sharpness 
and prominence of the umbonal ridge. The anterior end is much too small and 
short for W. rugatina, W. concentrica and W. scofieldi. In each case other differences 
might be mentioned, but those selected will, it is believed, suffice. 
~ Greatest length, 23 mm.; distance from beaks to posterior extremity, 22.5 mm.; 
posterior hight, 16.5 mm.; thickness, 18 mm. Antero-ventral—postero-cardinal diam- 
eter, 17 mm. 
Figure 23 is taken from a doubtful left valve, obtained from the lower Trenton 
in Jo Daviess county, Illinois. As viewed now, this specimen represents an ancestral 
form or variety of the present species from which also W. ventricosa, W. truncata and 
perhaps other species as well have been evolved. More and better material, however, 
is necessary before such a view of its relations can be consdered either as proved or 
disproved. 
Formation and locality.—The typical form is from the middle Galena near Wykoff, Minnesota. 
Waitetia ventRicosA Hall, 
PLATE XLI, FIGS. 24—26. 
Edmondia ventricosa HALL, 1847. Pal. N. Y., vol. 1, p. 155. 
¢ Not Palearca ventricosa, HALL, 1859. Pal. N. Y., vol. iii, p. 271, and Twelfth Rep. State Cab., 
pp. 10, 68 and 95. (?—Cyrtodonta huronensis BILLINGS.) 
Not Cypricardites ventricosus HALL, 1862. Geol. Rep. Wis., vol. 1, p 438; nor WHITFIELD; 1882. 
: Geol. Rep. Wis., vol. iv. p. 209. (=Cyrtodonta, sp., undet.) 
Believing that this species is represented among the undetermined fragmentary 
shells from Minnesota, I thought it well to give illustrations of authentic specimens 
from the Trenton of New York. These were received in an exchange some time 
ago. Quite recently I sent two of them to Prof. R. P. Whitfield, of the American 
Museum of Natural History, who compared them with the original types of the 
species and verified the identification. 
An examination of the New York examples established what I had already 
suspected from the original figures, namely, that the species is a true Whitella and 
not, as is commonly believed, a Cypricardites or Cyrtodonta. Its place in the genus 
will be seen at once, when compared with other species of the genus figured on 
plates xz and xr. The shell was thin, the beaks were full and prominent, the 
umbonal ridge, though not as sharply defined as in many other species of the genus, 
is still a more conspicuous feature than in any species of Cyrtodonta, the hinge has a 
narrow external ligamental area or escutcheon, and ridge-like supports for a posterior 
internal hgament, but no posterior lateral teeth. In short, the species presents 
