15G PALAEONTOLOGY OF ILLINOIS. 



growth, and showing, under a magnifier, indistinct granula- 

 tions. Some specimens have faint indications of two plica- 

 tions within the ventral sinus. Length, 0.38 inch; breadth, 

 0.35 inch ; convexity, 0.26 inch. 



As remarked by Prof. Swallow, this species resembles S. lincatus, Martin 

 (sp.), but differs materially in the possession of obscure radiating plications, 

 and in being more distinctly inequivalve. Young specimens also resemble 

 somewhat S. Urei, Fleming,* but they may always be readily distinguished 

 from that shell by their plications. 



Locality and position: Same as last. 



LAMELLIBRANCHIATA. 



Genus CAEDIOPSIS, M. and W. 



Cardiopsis, Meek and Worthen, June, 1861. Proceed. Acad. Nat. Sci., Philad., p. 144. 



Shell equivalve, somewhat inequilateral, very slightly ob- 

 lique, ovate or cordiform, entirely closed ; beaks rather eleva- 

 ted, distinctly incurved and directed towards the anterior side ; 

 surface marked by radiating strias or costse ; cardinal margin 

 short and rounding into the posterior border; hinge provided 

 with one or two distinct anterior teeth in each valve, near the 

 beaks. (Ligament and muscular impressions unknown.) 



In first describing the species we regard as the type of this genus, we placed 

 it provisionally in the genus Cardiomorpha, deKoninck, stating at the same 

 time that we suspected it to be generically distinct. Prof. Hall has since de- 

 scribed the same species, from the same locality, in the Thirteenth Annual 

 Report of the Regents of the University of New York, under the name of 

 Megamhonia Lyoni,f and mentions the presence of two strong anterior teeth in 

 the hinge of one valve. On clearing away the matrix from the hinge of one 



* Mr. Thomas Davidson, of London, after a careful comparison of specimens from 

 Illinois, states that he regards Ambocculia gemmula of McChesnej r , (which had been pre- 

 viously described by Dr. Shumard, in the Missouri Report, under the name of Spirifer 

 planoconvcxvs), as identical with Spirifer Urei of Fleming. 



f The genus Megamhonia was founded upon Pterinea cardiiformis, Hall, belonging 

 apparently to the Arcidx or Aviculidse. 



