41 American East Coast Arcas 



41 



Area idonea Conrad 



Plate IX, Figures 14, 15, 16, 17 



Arm idonea Conrad, Fos. Tert Form., p. 16, pi. i, fig. 5, 1832. 

 Ana stillicidium Conrad, 1. c, p. 15, pi. i, fig. 3, (young). 

 Area idonea Conrad, Fos. Med. Tert., p. 55, pi. 29, fig. 3, 1840. 



Seap/iarea (Area) idonea Conrad, Acad. Nat. Sci. I'hila., Proc. for 1S62, p. 579, 1S63. 

 Seapliarea (Seapharca) idonea Dall, Wagner Free Inst. Sci., Trans., vol. 3, pt. 4, p. 639, 1898. 

 Area (Scapharea) idonea Glenn, Maryland Geol. Surv., Miocene, pp. 387, 389, pi. 106, figs, i, 2, 

 1904. 



"Cordate, inequivalve, ventricose, and slightlj^ sinuous; ribs about 25, narrow and 

 crenulated; the crenulations most distinct on the larger valve; beaks very prominent and 

 distant; area with undulated grooves; hinge with the series of teeth contracted in the 

 centre, and a little dectirved at the ends. 



''Locality. St. Mar^^'s River. Md. Upper Tertiar3^" — Conrad, 1832. 



Shell large and heavy; ribs twenty- four to thirty-two; ribs on the middle of the shell 

 broad and flat and as wide as or wider than the interspaces, anterior ribs usually nar- 

 rower than the interspaces, posterior ribs rounded; the anterior ribs and the distal part 

 of the central ribs with a longitudinal sulcus, ribs about the umbonal ridge often with 

 two or more sulci, making them appear striated; interspaces about the umbonal ridge 

 sometimes striated; cardinal area wide, with four to eight, commonly five, concentric 

 grooves on the larger specimens; distal teeth usually irregular in the older specimens; 

 posterior end produced. 



This species is related to A. stamfnea from which it differs by its larger size and 

 more rounded form, the posterior side is more produced, the posterior and ventral mar- 

 gins more rounded, the umbonal ridge less angular, the hinge-line is proportionately 

 shorter, the ends of the hinge do not form a conspicuous angle with the anterior and 

 posterior margins, the shell is more evenly inflated, there is only a slight flattening or 

 none anterior to the umbonal ridge, in A. stamiiiea the ribs are mostly high and narrow 

 while in A. idonea they are broader and flatter, specimens of idonea from Maryland have 

 less anterior beaks, but this does not hold true for specimens from Florida. 



This abundant species has a wide variation in form. Conrad's type, from St. 

 Mary's River, is short and has only twenty-five ribs. The upper bed at Alum Bluff, 

 Florida, also contains a short form with twenty-five or twenty-six ribs. The ribs are 

 high and square. It resembles A. staminea in its little produced, nearly straight poster- 

 ior margin and beaks nearer the anterior end of the hinge than in the Maryland forms. 

 It is separated from A. staminea by its square ribs and rounded vimbonal ridge. 



The common variety from St. Mary's River, which was described by Conrad in 1840, 

 usually has twenty-nine or thirty ribs and is more produced posteriorly than the type. 

 It is also found at Alum Bluff, Florida. 



In the preceding varieties the shell is little, if any, higher posteriorly than near the 

 middle. A variety from the upper bed at Alum Bluff is elongated along the umbonal 

 ridge so that the greatest height of the shell is behind the middle. The anterior margin 

 curves outward a little from the hinge for a short distance, then evenly around to the 

 posterior margin, which is oblique to the hinge. There are twenty-four to twenty-six ribs 



