72 HY"ATT ON THE TERTIARY SPECIES 



form the extreme spirality observable in another view of a precisely similar specimen in 

 fig. 2, by the side of the first described. 



FOURTH SERIES. 



Flanorbis Steiuheimensis- 



Variety aequimbilicatus. 



PI. m. Steinheimensis var. aequiumbilicatus Hilg., Op. cit., fig. 1. 



Forms which are aequiumbilicated are quite rare in the lower formations of the Pits. 

 One is figured in section on line h, fig. 17, pi. 1, another on line a, fig. 2; almost all others 

 only approximate to this condition and ought perhaps to be placed with the inaequiumbili- 

 cated forms. They are the true transitions from the latter, but also possess the more 

 cylindrical or equal-sided and less involute whorls of the aequiumbilicated variety, such 

 are those figured on plate 1, line a, figs. 1, 3, 4, line c, figs. 1, 19, and line m, figs. 1-7. 



In the aequiumbilicated variety there are several sub- variations. Those Avith the mouth 

 and last whorl or whorls turned upwards, those with these parts central, and those 

 with these parts turned down as in fig. 7, line m, pi. 1. 



In the inaequiumbilicated variety there are similar variations in the direction of the last 

 whorls, but here the downward or spiral tendency is of course predominant. In the 

 aequiumbilicated variety I did not find a tendency to flatten the upper or lower sides, but 

 it must be taken into consideration that very few specimens of this variety were found. 



In the inaequiumbilicated variety, especially in the sub-variety with the mouth turned 

 upwards, there is a decided tendency to flatten the upper sides of the whorls and this is 

 correllated with a corresponding tendency to angulate or produce a ridge-like angularity 

 in the whorls on the outer side, both above and below and on the inner lower side near 

 the umbilicus. For convenience sake I have called these ridges the second, third, and 

 fourth carinae, reserving the designation of first carina for the innermost umbilical ridge 

 on the upper side, which appears so prominently in PI. discoideus. Forms may be picked 

 out which show this tendency^ in every way in the sub-variety with the mouth turned 

 downwards, though the second carina is very rarely seen. 



In some specimens only the third carina is seen, and these are remarkably similar to 

 PI. levis as figured by Sandberger. In others the fourth carina alone, or third and fourth 

 carinae with a slight flattening of the lower side occur, and the second and fourth carinae, 

 but in none did I find the third carina alone. 



The forms united under this name have, besides the characteristics above given, the 

 following : the whorls are more cylindrical, and the increase in the size of the whorls 

 by growth is less marked than in Steinheimensis proper, and therefore the involution a 

 trifle less. 



Variety Steinheimensis. 

 PI. Steinheimensis Rilg., Monatsb. K. Preuss. Akad. Wissensch,, Berlin, July 1866, 

 p. 485, fig. 2. 



PI. Steinheimensis Sandb., Op. cit., p. 644. 



1 It must be observed that I here speak of a hardly per- mouth and lower side of fig. 5, Hue m, pi. 1. 

 ceptible angularity such as is shown in the outline of the 



