FOSSILS OF THE JURASSIC PERIOD. 295 



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

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

 species of Cardium. Type 8. typica. 



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 lamellae of shell; an inner and an outer one, the 

 inner one originating- on the inside of the umbonal cavity, and rising to the 

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

 diverging considerably anteriox'ly, so as to leave a gradually- widening space 

 between the two, the anterior end of which is partitioned off, forming the 

 anterior muscular scar, or pit, and leaving a smaller, somewhat triangular, 

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

 recurved, hooked tooth rises, overarching the cavity beneath. In the left 

 valve, the two lamellse are less distinct; the whole cardinal border being 

 thickened, and the cavity into which the tootli of the right valve J&ts is 

 excavated in its substance, immediately in front or against the beak 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 CucuUcea 

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

 muscle as in those genera, and in the species S. typica 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 appearance of the shells would indicate a 

 much rhore recent date. 



Septocaedia typica n. sp. 



Plate VII, figs. 26-29. 



Shell small, subrhomboidal in outline, the united valves 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, slightly elevated posteri- 



