NATURAL SCIENCES OF PHILADELPHIA. 51 



teeth in each valve, which with other characters show a wide 

 generic difference between it and Pleuroconcha. 



PLIONEMA, Conrad. 



Shell snbrotnnd, sculptured with close radiating lines or fine 

 ribs ; hinge of left valve with 2 robust diverging teeth ; lunule 

 none. 

 Astarte Guerangeri, D'Orbigny. 



The radiating ribs and lunule are I think sufficient to distinguish 

 this as a subgenus. When it is considered that all the numerous 

 species, Cretaceous and recent, of the genus Astarte or Crassina 

 are characterized by a well-marked and generally profound lunule, 

 and when ribbed, always concentrically, the Astarte Guerangeri 

 forms too marked an exception to place it in the group of typical 

 species. 



SCAMBULA. 



Shell triangular, compressed ; in the right valve one direct tooth 

 under the apex, with a pit on each side of it, and a long lateral 

 tooth anteriorly, posterior dorsal margin carinated, which promi- 

 nent line fits into a doubled lateral tooth, in the opposite valve, 

 left valve with 2 long approximate direct teeth, and a long ante- 

 rior marginal lateral tooth, pallial line invisible, inner margin finely 

 crenulated on a raised line. 

 S. perplana, Conrnd, PI. I., fig. 2 



The crenulations on the shell closely resemble those of Pachy- 

 th;erus ( Cretaceous forms of Crassatella), being arranged in a 

 slightly prominent line. The hinge of this shell is very distinct r 

 from that of Crassatella. 



PTEEOMERIS. 



Shell triangular, compressed, radiately ribbed, not oblique ; 

 hinge of left valve with 2 diverging cardinal teeth, the anterior 

 one slightly grooved ; the posterior one elongated, profoundly 

 bifid ; posterior hinge margin carinated. 

 P. perplana, PI. I., fig. 3. 



This genus was indicated in the Proceedings of the Academy of 

 Natural Sciences in 1862. It was not founded on Astarte minu- 

 tissima as stated by Stoliczka, but on Cardita perplana a Mio- 

 cene shell. Micromeris was proposed for the former, which from 



1872.] 



