40 
BULLETIN OF THE LABORATORIES 
The two species associated in this genus differ so greatly that it 
may be seriously doubted if they can continue in the same genus. 
Unfortunately for the comparison, the right valve alone of D. 
halli appears to have been found, thus leaving open the question as to 
its being inequivalve. The hinge-line is straight and produced in this 
species, while it is quite inconspicuous and curved in D. whitei. The 
greater number of characters of Dexiobia, however, apply better to the 
second species, though in that case the name, as it proves, is less ap- 
plicable, some at least, of the species being nearly equivalve. 
In this predicament the best that can be done seems to be to re- 
gard “C. ovata” as the type of the genus Dexiobia with somewhat lim- 
ited characters, even though strongly suspicious that that species prop- 
erly belongs under Cardiopsis. The remaining forms from the Wav- 
erly must then constitute a new genus. 
A considerable suite of specimens, all from the upper part of the 
freestone enables us to prepare the following description. 
Shell of medium size, obliquely ovate in outline, quite convex, 
asymetrical. Right valve generally somewhat higher than long, vari- 
able, triangularly ovate, very gibbous ; anterior margin evenly rounded; 
lower margin ge ntly curved or somewhat straight ; posterior margin 
more acutely rounded, passing obliquely into the short hinge-line which 
bears the impress externally oi two teeth, one being anterior the other 
posterior ; beak very prominent, acute, extending above and arching 
over that of the opposite valve ; greatest convexity about at the middle 
of the surface which falls off rapidly in all directions ; surface orna- 
mented by nearly sixty rounded costae and irregular concentric striae 
and folds. Left valve obliquely ovate, produced posteriorly, slightly 
convex, with a short, obtuse beak ; post-umbonal plane at right angles 
to the valve ; surface marked as in the other valve. The valves meet 
closely. Greatest height 20 mm , length 18 to 20 mm. About 16 
costae to a centimeter in a specimen 20 mm. high ; in smaller speci- 
mens twice as many. 
It is indeed possible that two species are represented by our 
specimens. The greater number resemble Cardiopsis radiata in the 
surface markings but are certainly inequivalve and resemble D. ovata 
in outline more nearly than Cardiopsis. If Cardiopsis should be found 
to be inequivalve it would relieve us of the necessity for the additional 
generic term. The fact that the two forms are derived from the same 
horizon is suggestive. 
