190 MEMOIRS OF THE CARNEGIE MUSEUM. 



marginata. It frequents riffles, and this renders it the more conspicuous, since 

 there are few species on the Atlantic side which prefer this habitat. 



According to our present knowledge, A. marginata susquehannce is most 

 abundant in the Juniata River, and those parts of the Susquehanna system, which 

 are within the Allegheny Mountains. There is no trace of it in the Delaware or 

 Potomac drainages. 



The peculiar features of the distribution suggest the idea, that this form came 

 from the West by crossing the AUeghenian divide. This has been discussed else- 

 where (Ortmann, 1913a, pp. 370 et seq). 



Alasmidonta (Decurambis) varicosa (Lamarck) (1819). 

 Alasmidonta marginata Simpson, 1900, p. 670; Alasmidonta varicosa (Lamarck) 

 PiLSBRY, 1901, p. 17; Alasmidonta varicosa (Lamarck) Simpson, 1914, p. 506. 



Plate XII, fig. 5. 

 Records from Pennsylvania: 



Lamarck, 1819 (Schuylkill River, Philadelphia). 



Lea, Obs. II, 1838, p. 56 (Crum Creek, Delaware Co.)''' 



Bruckhart, 1869 (Lancaster Co.). 



Hartman & Michener, 1874 (Chester Co.) 



Marshall, 1895 (Philadelphia). 



Schick, 1895 (Tohickon Creek, Bucks Co.; Neshaminy Creek, Bucks Co.; Munckinipattus Creek, 



Glenolden, Delaware Co.) 

 Ortmann, 19096, p. 207. 

 Caffrey, 1911 (Delaware River, Northampton Co.) 



Characters of the shell: The shell of this species differs from that of A. fnarginata 

 by its smaller size, and by the outline, which is rather more subovate than sub- 

 trapezoidal, and slightly more elongate on the average. This difference of the 

 outline is brought about by a different development of the posterior ridge and 

 posterior slope. The posterior ridge is not so sharp, more broadly rounded, and 

 often appears biangulate,'^- and the posterior slope is not so distinctly truncate. 

 This renders the whole posterior section of the shell more elongated and rounded, 

 and in consequence the anterior section is comparatively less developed, so that 

 the beaks are situated more anteriorly, generally at a distance from the anterior 

 end of considerably less than one-third of the length of the shell. 



The color of the epidermis is much like that of A. marginata, with the darker 



"' By a mistake I have given tliis record (Margarildna marginata) under Strophilus undulatus 

 (Ortmann, 19096, p. 205). 



"^ Indications of the biangulate character is sometimes found also in the western .4. marginata. 



