FOSSILS OF THE JURASSIC PERIOD. 295 



Surtace of the shell marked, in the typical species, by strong, elevated, 

 radiating ribs, with ornamented snrfaces similar to many of the recent 

 species of Canlmin. Type »S'. tupica. 



The shells for which the above generic name is proposed are small, 

 few specimens of the typical species exceeding three-eighths of an Inch in 

 their greatest diameter. Externally they closely resemble the Eocene forms 

 of Cardita, but the hinge-structure separates them at once from all other 

 known forms. The cardinal border of the right valve appears almost as if 

 composed of two distinct lamelhi? of shell, an inner and an outer one, the 

 inner one originating on the inside of the umboual cavit}-, and rising to the 

 level of the valve margin, coalescing with the outer portidu posteriorly, and 

 diverging considei'ably anteriorly, so as to leave agradunlly-wideniiig space 

 between the two, the anterior end of which is partitioned off, foi-ming the 

 anterior muscular scar, or pit, and lea\ing a smaller, somewhat triangular, 

 cavity posterior to it. From the inner lamella bounding this cavity, the 

 recurved, hooked tooth rises, overarching the cavit}' beneath. In the left 

 valve, the two lamelkx'. are less distinct; the whole cardinal bord(_'r bt-ing 

 thickened, and the cavity into which the tootli of the right valve fits is 

 excavated in its substance, immediately in front or against tlie )»eak of the 

 valve. There are no lateral teeth in either valve. The septum bounding 

 the inner margin of the anterior muscular scar is similar to that of CucuUcra 

 or Idonearca, except that it borders the anterior instead of the posterior 

 muscle as in those genera, and in the species S. tijpka reaches almost to the 

 basal margin of the valve. 



The type-species occurs in rocks referred with some doubt to the 

 Jm-assic, although the general appeai-ance of the shells would indicate a 

 much more recent date. 



SePTOCARDIA TYl'ICA 11. sp. 

 I'latcVII, M.^s. '20--J9. 



Shell small, subrhomboidal in outline, the united vah'cs angularly cor- 

 diform and very gibbous. Valves longer than high, oblique, and widening 

 posteriorly; beaks strong, angularly tumid and enrolled, situated near the 

 anterior end of the shell ; cardinal line arcuate, slightl}' elevated j)o.><teri- 



