362 SYSTEMATIC PALEONTOLOGY 



Order PRIONODESMACEA. 



Supernimily MYTILy^EA. 

 Family MYTILID^. 



Genus MYTILUS Bolten. 



Mytilus conradinus d'Orbigny. 

 Plate XCVI, Figs, la, lb. 



Mytilun incrasmtua Counid, 1841, Amer. Jour. Sci., vol. xli, il 347; uot of Deshayes 



1830. 

 Mytilus incrassatus Conrad, 1845, Fossils of the Medial Tertiary, p. 74, pi. xlii, lig. 4. 

 Mydlus Conradinus d'Orbigny, 1852, Prod. Pal. Strat., vol. iii, p. 127. 

 Mytilus incrassatns Tuomey and Holmes, 1856, Pleioceue Fossils of South Carolina, 



p. 32, pi. xiv, figs. 1, 2. 

 Mytiloconcha inn-assata Conrad, 18(32, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 291. 

 Mytiloconcha incrassata Conrad, 1S63, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 579. 

 Mytiloconcha incrassatn Meek, 18(54, Miocene Check List, Smith. Misc. Coll. (183), 



p. 7. 

 Mytilus Conradinus Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, pt. iv, p. 787. 



Description. — "Thick, much inflated; anterior margin slightly in- 

 curved near the middle ; basal margin not obtusely rounded ; hinge thick, 

 with slightly prominent robust teeth." Conrad, 1841. 



A remarkably large and almost perfect left valve from Plum Point 

 shows the following characters: Shell highly convex, apically acute, lat- 

 erally curved and posteriorly rounded in outline ; external surface marked 

 by distinct, inequidistant, concentric undulations with finer subordinate 

 ones between; dorsal margin a convex curve; ventral margin a gently 

 concave curve; beak heavy, solid; posterior portion of shell moderately 

 thin ; interior dull pearly ; hinge or tooth ridge long, narrow, curved and 

 prolonged on the beak as a marginal groove ; beak not medially grooved. 



The apical portions are usually the only part preserved. When young 

 and badly worn, as is often the case, it becomes very difficult to separate 

 M. conradinus from M. inciirvus. 



Length, 180 mm. ; width, 78 mm. ; diameter, 3-1 mm. 



Occurrence. — Choptank Formation. Governor Run, Flag Pond, 

 Jones Wharf, Peach Blossom Creek, Dover Bridge, Greensboro. Calvert 

 Formation. Chesapeake Beach, 3 miles south of Chesapeake Beach, 

 Plum Point, Church Hill. 



