TRANSACTIONS OF WAGNER 



828 



TERTIARY FAUNA OF FLORIDA 



retire is a comparatively thin coating over a stony or rocky layer which they 

 cannot pierce, the tendency in Panopca, My a, etc., is for relatively short and 

 broad shells, with shorter siphons, to survive; which naturally have a wider, 

 shorter, and more rounded pallial sinus, and shorter and more incurved 

 nymphs. I believe the influence of the environment is direct and not selec- 

 tive ; at all events, the association of situs and specimens so characterized is, 

 as far as I have been able to determine, quite uniform, whether selective or 

 not. 



It is extremely puzzling to endeavor to determine, out of a very large 

 series of specimens from one locality, how far the interchangeable variations 

 which present themselves can be regarded as differential in a systematic sense; 

 and, in the end, one has to rely more on a general habit of growth recogniz- 

 able with experience, but difficult to diagnose in language which shall not be 

 more prescriptive than the actuality. There is no difficulty in making a de- 

 scription ; the trouble is, as in so many other cases, to make a differential yet 

 just and impartial diagnosis. 



In addition to the differences more or less evidently due to situs there are 

 a series of differences which occur among specimens of a single species from 

 apparently the same situs, both in the fossil anil recent forms. These include 

 a nearly rectilinear as compared with an arcuate hinge-line, and a short as 

 opposed to a long insertion of the ligament. The length of the ligament is 

 perhaps coordinated with the heaviness of the valves, but the differences 

 alluded to occur so constantly that I have been led to suspect that they might 

 be due in part to differences correlated with sex in this genus. 



From the Chickasawan Eocene Harris has described (as Glycymcris, Bull. 

 Pal., ix., p. 69, pi. 13, fig. 16, 1897) P. alabmna, a species which he had pre- 

 viously (Proc. Acad. Nat. Sci. Phila. for 1896, p. 475, pi. 22, fig. 4) regarded 

 as a variety of P. porrcctoidcs Aldrich (Hull. Ala. Geol. Stirv., i., p. 37, pi. 4, 

 fig. 3, 1886), which was described from nearly the same horizon. Conrad 

 (Proc. Acad. Nat. Sci. Phila., iii., p. 290, 1848, and Journ. Acad. Nat. Sci., 2(1 

 Ser., i., p. 121, pi. 13, fig. 12) has described P. oblongata from the Vicksburgian. 

 This is distinguished from the similar P. clongata Conrad (Trans. Gcol. Soc. 

 Penna., i., p. 339, pi. 13, fig. I, 1835; and Am. Journ. Sci, 2d Ser., i., p. 

 214, pi. ii., fig. 2, 1846; not of Rocmer, Vcrst. Oolith., p. 126, pi. 8, fig. i, 

 1836) of the Eocene of Maryland, Virginia, and the District of Columbia by 

 its more anterior and prominent umbones. The former also occurs in the 

 Jacksonian. 





