rs PALAEONTOLOGY OF NEW-YORK. 



Genus Stroimiodoxta ( Hall). 



The Genus Strophodonta was originally proposed for strcphomenoid 

 shells, having a crenulated hinge-line, without foramen in the area of 

 the ventral valve. The internal characters were not at that time fully 

 known, and therefore could not be included in the description. 



In S. demissa, which was regarded as the type of the genus, the area 

 of the ventral valve is free from any indication of foramen ; the dental 

 lamelltB are partially or essentially obsolete, and the divaricator muscular 

 impressions are spreading flabelliform without limitation by an elevated 

 ridge. In the dorsal valve, the cardinal process is bifurcate from its origin, 

 and is directed backwards beneath the area of the ventral valve. On the 

 lower side of this ventral area there is a bilobed callosity or process 

 which is more or less prominent and embraced by the divisions of the 

 cardinal process of the opposite valve ; thus strengthening the articulation 

 of the valves, or assisting in keeping them in position. The divisions 

 of the cardinal process often extend so far beneath the area as to form 

 conspicuous pits in the substance of the shell within the umbonal cavity. 



The valves are so closely articulated by the interlocking of these pro- 

 cesses, that the separated ventral valves are usually broken in the mid- 

 dle of the area : this happens also to other species, and it is compara- 

 tively rare to find the inner margin of the ventral valve entire. This 

 accident is probably caused by the pressing or crushing of the dorsal 

 valve into the cavity of the ventral valve, while the central portion of 

 the two, remaining firmly united, carries away the middle of the ventral 

 area, which is thinner on each side of the median. 



In the S. reversa of Iowa, this bilobed process of the ventral area is 

 very conspicuous ; and in some specimens of this and of S. Icpida, the 

 central process remains very prominent, while the margin of the area 

 on either side is indented by the branches of the cardinal process of the 

 opposite valve. In these species we not only have no evidence of trian- 

 gular foramen, but there is an absolute thickening on the lower side and 



