- 108 - 



Remarks .- Several valves are referred to this species, among 

 them one which is not tj/pical since it is more produced posterior- 

 ly and the cardinal margin is longer hehind the umbo. It is to 

 he expected, however, that so irregular a form should be variable. 



A. inusitata may be recognized by its irregular, distortei 

 shape, prominent byssal gape, e::cavated posterior slope and numer- 

 ous, fine raf''ials. Several closely relate:! species have been des- 

 cribed from various localities. A. irregrularis Dall (l), a Tampa 

 silex and Jaloosahatchie form, is larger, with njnbonos that are 



(1). Trans. Was-ner I?ree Inst. 3ci., Philaf^elphia, vol. 3, pt. 

 4, pp. 623-624, pi. rs, f iff . 5, l',398. 



more posterior in position and coarser ribs. A. marylan^i?a Jon- 

 rad (2), from the Oligocene of 'Florida and the KiOcene of i.'.sry- 



(2). Fos. i:ed. Ter. U. 3., p. 54, pi. 29, fi^. 1, 1040/ 



land, is larger, vath lov/er, less inflated umbones, a broad, low 

 posterior ridge and an unexcavated posterior slope. The Tampa 

 silex A. arcula Heilprin iz) is higher, more inflated, less regular 



(-?). Trans. Wagner ?ree Inst. 3oi., Philadelphia, vol. 1, p. 118, 

 pi. 16, fig. 65, 1887. 



