ORTMANN: monograph of the naiades of PENNSYLVANIA. 155 



so high, so that the shape becomes short-ovate, with the [)OSterior end nar- 

 rower. 



A. virgulnta. {Type locality North CaroUna.) In shape intermediate between 

 A. williamsi and A. tryoni, but with brighter coloration and more distinct raj'S. 

 This is, according to Simpson (1914, p. 388) a southern form of A. catarada. 



Such short and high shells seem to be rather scarce in Pennsylvania, but the 

 tryoni-type occurs in the larger rivers (Delaware and Schuylkill), and might be 

 called characteristic of this environment. 



The thickness of the shell is also variable in A. catarada; the pond-forms 

 being generally thinner, while the creek-forms are thicker. A. catarada is also 

 quite variable in the convexity of the valves; and there are more compressed and 

 more inflated shells, both in ponds and creeks. The inflation, if present, is re- 

 stricted to the disk, and does not extend to the umbos, and forms having inflated 

 beaks, like A. grandis footiana, are not known among the variations of A. catarada, 

 except possibly in the South, among the virgulata-typo, in which there seems to 

 be a tendencj^ in this direction. 



The various forms of A. catarada, described above, are all connected bj' inter- 

 grades, and they seem to be special reactions to special environmental conditions, 

 although we are not in all cases sure what are the essential features of the environ- 

 ment, which are active. In the Schuylkill Canal I found, for instance, shells 

 resembling the creek-form (but thin-shelled), where we should rather expect the 

 pond-form. But here the bottom of the canal was somewhat stonj^ with very 

 little mud, and it is quite possible, that in this case the character of the bottom was 

 all important. Specimens from Echo Lake, Monroe Co., more closely resemble 

 the creek-form. Since these various forms turn up all over the range of the species, 

 they are to be regarded simply as variations, not as varieties. The idea, that they 

 may be species is entirelj^ excluded. 



I should add here a few words about a peculiar, distorted form, received from 

 Mr. C. H. Conner from the Delaware River, Newbold, Gloucester Co., New 

 Jersey. There are five specimens, resembling in general characters the tryoni- 

 type, but all possessing a peculiar radial rib, running from the beaks toward the 

 lower margin of the shell, and in front of the rib the shell is somewhat compressed. 

 It seems as if the growth of the shell in its anterior ])art had been checked or re- 

 tarded. The specimens look decidedly like cripples, and I take them for such, 

 j'et it is remarkable that a number of specimens showing the same deformation 

 should be found at the same locality. 



