84 MOLLUSC A. 



formed two other genera of borers. The first he terms Ru- 

 PICOLA, having a transverse inequilateral shell) a little 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 Linnaean 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 aequivalvis, binaequilatera 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- 

 teme des Animaux) 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 VENEBICARDIA, formed for the reception 

 of some fossil species, is thus defined : " Testa bivalvis, 

 aequivalvis, inaequilatera, extus longitudinaliter costata. Den- 

 tes cordinales sub-bini crassi oblique secundi." The num- 



