ORTMANN: monograph of the naiades of PENNSYLVANIA. 29 



Characters of mricty: This form diffcuvs from plicala of Lake Erie by being 

 considerably larger, attaining a huge size; in fact it is one of our largest and heaviest 

 shells. The shell is generally rather flat, with the diameter less than forty percent 

 of the length, and the posterior wing is well-developed, so that the upper margin 

 appears elevated posteriorly. The beaks are very slightly prominent. The surface 

 sculpture is similar to that of plicata. In large specimens there are upon the sides 

 four or five transverse bars, but they are, as a rule, more distinct and broader 

 than in A. plicata. The radiating ribs upon the posterior slope and the wing are 

 more frequentl}^ and more distinctly developed. Color of epidermis generally 

 darker, although young shells are often as light as plicata. The growth-lines are 

 less regular, and old shells become uniformh' black. Nacre white, often beauti- 

 fully iridescent posteriorly. No difference between the male and the female in 

 the shell. 



L. H. D. Pr.ct. 



Size: 1. Jamestown, Cat. No. 61. .3831 161 mm. 12.3 mm. 68 mm. .42 



2. Mahoningtown, Cat. No. 61.38.30 99 " 78 " 38 " .38 



No. 1 is the largest specimen on hand. 



Soft parts and Glochidia (See Ortmann, 1912, p. 246). 



Breeding season: TypicalUy tachytictic, breeding from May to July. I found 

 gravid females on the following dates: May 13, 1911; May 17, 1910; IMay 23, 

 1911; May 23, 1912; INIay 23, 1914; May 24, 1911; June 19, 1909; July 3, 1908; 

 July 8, 1909; July 10, 1909; July 10, 1911. Glochidia were seen only in July, 

 and a discharging specimen M'as observed on Julj' 10, 1911. Wilson & Clark 

 (1912a) report gravid specimens as late as July 28. 



Remarks: Amblema plicata costata undoubtedh^ is the parent form, from which 

 A. plicata is derived as a depauperate form in Lake Erie. The former is rather 

 variable in some of its characters, but it is always much larger, generally more 

 compressed, and the posterior wing is more elevated. The sculpture varies a 

 good deal. I have specimens in which the shell is practicality smooth, without 

 any folds (Compare the figures of Baker, 1898a, PL 12, fig. 1 ; Smith, 1899, PI. 82, 

 and our figs. 2 and 3 on plate III). Sometimes the folds are peculiarly developed, 

 showing a tendency to be divided into nodes. I have even a specimen in which 

 these nodes form irregular vertical ribs, running toward the lower margin of the 

 shell, so that, together with the ribs of the posterior slope, a sj^stem of low bars 

 is indicated, which diverge from the posterior ridge of the shell in the direction of 

 the lower and posterior margins. 



The specimens from Pennsylvania are all to be regarded as true costata, and I 

 have not seen any forms which incline toward the western and southwestern 



