66 AMERICAN MIDLAND NATURALIST 
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writers on the genus have subsequently given practically the 
same definition. However, within this genus there are a number 
of notable departures from the typical generic dentition, as ex- 
emplified in the type, Lampsilis ovata (Say) Rafinesque, and in 
the groups of L. (Proptera) gracilis, L. (P.) amphichena, and L. 
(P.) leptodon, we find species having the pseudocardinals, laterals, 
often rudimentary or almost wanting. 
In the accompanying figures are shown the normal right 
valve of L. ligamentina (Fig. 1.) and a right valve of the same 
species in which there is a strongly developed third pseudocardinal. 
(Fig. 2). Each valve in this species has normally two pseudo- 
cardinals, with two laterals in the left, and one in the right valve. 
The pseudocardinals on the left valve are nearly equal in size, 
and are solid, triangular, and marginally crenulate. In the right 
valve the anterior pseudocardinal is very small (see figure); the 
posterior is large, triangular, and crenulate. The laterals are 
strong and direct and are curved post-ventrally, and show on 
