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 \ve have no means of determining with much confidence to wlrit 



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

 species of the above group, and the ca-t shows an impression behind the beaks, 

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



>peeies (if TiilX-i-iiliit. 



I.<xii/iti/ ami jM*t(l<m. Jurassic heds at southwest base of the Black 1 1 ills. (No. 

 298.) 



FAMILY CARDIIDJ3. 



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

 cli.srd or raping posteriorly, crenate or dentate within ; surface generally 

 with radiating- rost.-i 1 , or variously sculptured, sometimes smooth. Hinge 

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

 exU-riiul. 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, 

 palpi sk'iidiT 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 

 I.iiK-iii-i/ini/i), Coivnlum, and Papyridea, The species constituting the recent genus 

 Adacna (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 Lithocardium, and several unnamed extinct genera, also belong 

 here. The remarkable palaeozoic genus Conocardium 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? Rudisicu. Although not prepared to adopt this conclusion, we are by no 

 means clearly satisfied that it belongs properly to the Cardiidae. 



Genus PROTOCARDIA, BERYCH. 



SynoH. Cardium (sp.), SOWEBBT, D'OBBioirT, and others. 



Protocardia, BKTRICH, Zeitschr. f. Halak. 1845, 17. GBIKITX, Grnndr. d. Vergt. 1846, 421. COHBAD, Report 



Hex. Bound. Survey, 1858, 150. MEEK, Smithsonian Check List North American Cret Fossils, 1864. 

 Elym. ir^rrot, first ; Cardium. 

 Type. Cardium THIlnnum, SowERBT. 



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

 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 strioe on the sides and front of the valves, and radiating rostu- behind (the 



13 December, 1804. 



