Maryland Geological Survey 437 



of which, when entire, is completed by two small deltidial pieces. Dorsal 

 valve flat or slightly convex, the front and lower lateral margins abruptly 

 inflected: beak straight; hinge-line of the ventral valve more declining, 

 with border often grooved for the reception of the edge of the dorsal valve. 

 The hinge-line of closed valves often ornamented by fibers or fimbria, 

 forming a byssus-like appendage. 



" Surface marked by about eleven or twelve rounded or subangular pli- 

 cations on each valve, two of which, in the middle of the ventral valve, are 

 more prominent and slightly larger than the others; and between these 

 there is a third (usually) smaller depressed plication, with a shallow 

 sinus below the middle of the shell. On the dorsal valve the two central 

 plications, below the middle, are a little stronger and more widely sepa- 

 rated from the lateral ones, rising in front into a slight mesial elevation. 

 When seen from the front, the margin of the shell presents a very distinct 

 sinus, with a corresponding projection of the opposite valve, the entire 

 margin being deeply serrate. The finer surface markings are mostly 

 obliterated, some imbricating lines of growth alone remaining. The 

 fimbriated appendage along the hinge-line has been observed in so many 

 specimens, that I can only suppose it to belong to the animal economy. 

 The remains of this appendage are sometimes observed upon the inner 

 margins of the separated valves." Hall, 1859. 



A. fimbriata is undoubtedly a local variation of the very widely dis- 

 tributed A. flahellites, but is a good species and may be recognized by the 

 following characters : A. fimhriala never attains the size of A. flabelUtes, 

 the plications are less angular and more depressed, causing the ventral 

 sinus to be nearly obsolete and the two median plications of the dorsal 

 valve less elevated anteriorly. The dorsal valve is flat with the lateral and 

 anterior margins narrowly but rapidly rounded to meet the ventral shell. 

 The cardinal angles are also more rectangular, and this becomes a pro- 

 nounced feature when the cardinal fimbria are preserved. 



The posteriorly directed cardinal fimbria are a peculiar feature of this 

 species and are unlike any other outgrowth similarly situated in brachio- 

 pods. They are developed from both valves and each plate is concave to 



