84 MOLLUSCA. 
formed two other genera of borers. The first he terms Ru-. 
PICOLA, having a transverse inequilateral shell, alittle gap- 
ing at the ends; no teeth or callosities. In an internal 
projection of each valve, there is a pit for the ligament. 
The other genus is named Saxicava. It is transverse, 
inequilateral, and gaping, without teeth, or callosity, or pit. 
The ligament is external. 
10. Venus. This Linnean genus contains so many 
species, that there is considerable difficulty in studying it. 
The formation of new genera, from its members, by dimi- 
nishing their numbers, must prove highly acceptible to the 
student of conchology. Lamark has succeeded so far by 
previously restricting the character of the original as fol- 
lows: “ Testa bivalvis zequivalvis, binzequilatera transversa 
vel suborbicularis. Dentes cardinales tres in utroque val- 
va, ad nates basi convergentibus. Ligamentum externum, 
nymphas labiaque obtegens.” The three diverging hinge 
teeth constitute the essential character of the genus, so that 
Lamark has been able to form three other genera from dif- 
ferent characters. 
The genus CyTHEREA (the meretrix of Lamark’s Sys- 
téme des Animauz) is thus characterised: ‘“ Cardo dentibus 
duobus tribusve approximatis, basi convergentes ; uno soli- 
tareo remotiuscula sub ano.” It must be confessed that the 
insulated teeth under the lunule, in the absence of other 
characters, is obviously artificial This genus contains 
many species requiring subdivision. 
The genus VENERICARDIA, formed for the reception 
of some fossil species, is thus defined: “ Testa bivalvis, 
aquivalvis, inzequilatera, extus longitudinaliter costata. Den- 
tes cordinales sub-bini crassi obliqué secundi.” The num- 
