76 CO'i^GRYUA—DITHYRA. 24. 



Shell two inches and a half long, and nearly three broad, thin 

 brittle, dirty white with a rufous tinge, and marked with regular 

 pale testaceous rays : beaks prominent, rather oblique : hinge with a 

 depression in the middle, and the rudiment of a small oblique denti- 

 cle, with a small lateral tooth on each side. 



We are unacquainted with this shell, and have consequently no 

 means of determining its proper place in our present arrangement. 



24. GOOBAUAA—TURTON. 



Testa trigona, cequivalvis, inoequilateralis, clausa. Cardo valvoe alterius 

 dentibus duobus cum fovea trigona interpositd, alterius dente unico : 

 lateralibus obsoletis. Ligamentum internum. 



Shell triangular, equivalve, inequilateral, closed. Hinge with two 

 teeth in one valve and a triangular cavity between them, in the 

 other valve a single tooth: lateral teeth obscure. Ligament 

 internal. 



The minute shells of this genus differ materially from the Mactra, 

 both in the form and disposition of the teeth ; the lateral ones con- 

 sisting of a rather remote simple tooth in each valve on the more 

 produced side : the valves are also closed. 



