334 Prof. T. R. Jones on 
under notice. On the other hand, there are many small indi- 
viduals of the subovate shape among specimens from Goth- 
land. Taking every thing into consideration, I am inclined 
to think that the oblong specimens may be the males of the 
species. In aspect and structure the thick, smooth, brown 
carapaces of the two sorts (oblong and ovate) are remarkably 
similar. Nevertheless, for the convenience of collector and 
statist, as the difference of shape is easily recognized, it may 
be allowable to follow Fr. Schmidt in using the name Hs7n- 
gert for the ovate and balthica for the more oblong form, 
even if it has not a strictly specific claim. 
Aiming at exact results, and trying to avoid unnecessary 
confusion in the endeavour to determine specifically either 
gradational forms or casts and damaged specimens (valuable 
on account of either rare occurrence or their geological posi- 
tion), { have taken pains in tabulating the measurements of 
my specimens ; but the numerical value of relative dimensions 
has not given me precise grounds for classifying them, and I 
have had to rely more especially on the usually recognized 
features and characters. 
Characteristic Features of the Leperditiade * 
1. Relative dimensions: absolute and proportional. 
i, Length of valves. 
i. Length of hinge-line. 
il. Height of valves. 
iv. Convexity of valves or thickness of carapace; often 
difficult to determine. 
In comparing the heights of valves, it is to be remembered 
that the right is the overlapping and therefore the higher of 
the two valves. 
2. Shape or outline. 
All approach the oblong in outline, with one (upper or 
dorsal) margin straight for greater or less extent of hinge- 
ment; but the Leperditie are subovate, with a more or less 
elliptical ventral curve. 
i. Approaching oblong, with relatively long hinge-line. 
ii. ” ovate, ” ” ” 
iil. > 7 op short: a. 
iv. Obliquely os 3 lone 
fe Bom) ” ” short ,, 
vi. Cylindroid, a koe 5, 
* T. R. Jones, “ Palewoz. Biv. Entom.,” Geol. Assoc. Proceed. 1869, 
pp. 2 and 9; Monthly Microse. Journal, Oct. 1, 1870, pp. 187 and 190. 
