68 HYATT ON THE TERTIARY SPECIES 



No very decidedly diseased or aged forms were observed, but the spiral is often 

 slightly irregular. 



The peculiar gibbous aspect of the whorls in fig. 7, line 6, is exaggerated by the 

 photograph, as it is also in fig. 1, on the same line, and may mislead the observer 

 into the belief that he is looking at specimens of "^^(S?"*- This, however, can be readily 

 corrected by comparing them with true ""'tevT'i on pi. 6, lines a, b. 



The apparent contrast between the two figures above mentioned, on line b, pi. 5, and 

 the two corresponding figures of PI. levis, on line a, is hardly perceptible in the 

 specimens themselves, which are really very large specimens of PL parvus. 



THIRD SERIES. 



Planoibis oxystomus. 

 levis 



The shells figured on pi. 1, line m, figs. 10-14, approximate to, but are still readily 

 separated from those figured on pi. 1, line a, figs. 12, 13. These last are slightly different 

 from the PI. ""^S""", from Undorf, but figs. 14-16, which are of about the same size, 

 would not be separated by the most conservative naturalist, if found at Undorf. The 

 full-grown forms, figs. 12, 13, line a, pi. 1, are larger, and approximate to true PI. oxystomus, 

 pi. 3, figs. 8, 9, line k ; in fact, they are so nearly identical with these that the young alone 

 show their afiinity to PI. ""'•'I'^lf''^ . The young have the shallower umbilicus on the lower 

 side, and that side of the outer whorl is flatter at the same age than in true PI. oxystomus. 

 Fig. 12, line a, pi. 1, even shows the peculiar mouth-run and general outline of 

 PI. oxystomus. 



There is one noticeable characteristic in the shells of PL Steinheimensis, Avhich 

 may also be cautiously used in separating the varieties just described, from those 

 of PL oxystomus. The striae of growth are not so decisively marked in Steinheimensis. 

 Shells with striations as prominent or as deejjly incised as is usual in PL oxystomus are 

 rare. The surface of the former has a smoother aspect than that of the latter, and 

 this is almost invariable in the young, while the young of many varieties of PL oxys- 

 tomus have very distinct striae. But though rare, such shells do occur, and some of them 

 are found in the intermediate varieties just described. 



Planorbis oxystomus. 



PL oxystomus Klein, Jahresh. Wurtt, Vol. 2, pi. 1, fig. 27. 



PL m. oxystomus Hilg., Op. cit., pi. fig. 7. 



PL m. revertens Hilg., O/). eit., pi. fig. 8. 



PL m. suprem,us Hilg., Op. cit., pi. fig. 9. 



Carinifex oxystomus Sandb. Op. cit., p. 643, pi. 28, fig. 3. 



Variety revertens Hilg. ^=^ ""•'ilrZ^^ pars. 

 After prolonged comparisons I am unable to find any characteristic by which this vari- 

 ety in some of its forms can be separated from the narrow umbilicated forms of PL 

 "XT'"'- These, like those figured on line k. fig. 6, and Ynw p, figs. 10-11, pi. 3, have shal- 

 lower umbilici on the lower side, wjth whorls less gibbous than is usual, and a mouth 

 which neither flares nor contracts, and has a very thin inner lip. 



