REPTILIAN AGE. JURASSIC PERIOD. 97 



The specimens of this species in the collection are casts, in a rather soft yellowish 

 sandstone, showing neither the hinge nor the muscular and pallial impressions 

 Consequently we have no means of determining with much confidence to what 

 genus it properly belongs. In form and general appearance it resembles some 

 species of the above group, and the cast shows an impression behind the beaks, 

 such as would be left by a posterior tooth or callus similar to that seen in many 

 species of Tancredla. 



Locality and position,. — Jurassic beds at southwest base of the Black HiUs, (No. 

 298.) 



Family CAEDIIDJE. 



Shell free, regular, equivalve, usually cordiform and gibbous ; margins 

 closed or gaping posteriorly, crenate or dentate within ; surface generally 

 with radiating costfe, or variously sculptured, sometimes smooth. Hinge 

 more or less variable, usually with cardinal and lateral teeth ; ligament 

 external, short and prominent. Pallial line simple, or slightly sinuous. 



Animal with mantle margins open in front ; siphons very short, dis ■ 

 tinct, and furnished along the sides and bases with tentacular filaments; 

 jDalpi slender and pointed. Gills two on each side, connected together 

 behind. Foot very long, bent or geniculate. 



The recent genera usually included in the family are Cardium, Lcevicardium (or 

 Liocardium), Corculum, and Papyridea. The species constituting the recent genus 

 Adaena (including Monodacna and Didacna), sometimes placed in this family, .seem 

 to belong to a distinct group, on account of their elongated, plain, and united 

 siphons, and their shorter compressed foot and deeply sinuous pallial line. 



The Jurassic and Cretaceous group Protocardia, the Cretaceous Liopistha, the 

 curious Eocene Litliocardium, and several unnamed extinct genera, also belong 

 here. The remarkable palaeozoic genus Gonocardium is likewise often referred to 

 this family, but its distinct coarsely prismatic cellular shell-structures has led some 

 naturalists to think it may even belong to the very widely removed, anomalous 

 order? Rudistcu. Although not prepared to adopt this conclusion, we are by no 

 means clearly satisfied that it belongs properly to the Gardiidce. 



Genus PEOTOCAEDIA, Berych. 



Si/non. — Cardium (sp.), Soweket, D'Oebtony, and others. 



Protocardia, Beykich, Zeitsehr. f. Malak. 1845, ]7. — Geinitz, Grundr. d. Verst. 1846,421. — Conrad, Report 

 Mex. Bound. Survey, 1858, 150. — Meek, Smithsonian Check List North American Cret. Fossils, 1864. 

 Etym. — wpoiTof, first ; Cardium. 

 Tijpe. — Cardium Hillanum, Sowekbt. 



Shell globose-cordate, closed all around ; subequilateral and but slightly oblique. 

 Hinge with one or two cardinal teeth, and usually one anterior and one posterior 

 lateral tooth, in each valve. Surface ornamented with very regular concentric 

 costse or strife on the sides and front of the valves, and radiating costee behind (the 



13 December, 1864. 



