58 HYATT ON THE TERTIARY SPECIES 



IV. Desckiptions op Series and Sub-Seeies. 



FIRST SERIES. 



Flanorbis minutus. 



Planorhis multiformis minuhis Hilg., Monatsber. d. Akad. d. Wissensch., Berlin, July, 

 1866, £ 15. 



PI. Zietenii (pars) Sand. Land und Siissw. Conchyl. d. Vorwelt., p. 645. 



PL hemistoma Klein, Jahreshefte Ver. Naturg. Wiirttemb. 2 vol. 1847, pi. 1, fig. 25. 



This remarkable species seems to have in the aspect of the whorl, and the general 

 thinness of the disc-like form a very close afl&nity for PI. crescens. It differs, 

 however, in the greater involution of the whorls in the healthy forms which precisely 

 resemble PI. Kraussii, in the aspect of the umbilici when viewed from above or 

 below. This part is narrower, and the internal whorls less exposed than in PI. crescens. 

 It is plain that a flattened form of PI. Kraussii would be precisely intermediate between 

 these two. After much search, I found a specimen which was a trifle flatter than 

 the usual square form, the outline being similar to that of PI. minutus, but it was 

 still considerably larger than the ordinary specimens of this species, and could not be 

 considered a hybrid. A close comparison between the largest minutus and PI. parvus, was 

 more successful. The typical minutus forms have a narrow umbilicus, as compared with 

 PI. parvus, in any of its varieties, but this characteristic is exceedingly variable in 

 the species, and many of the specimens have a wide umbilicus on the lower side. If we 

 compare these with the young of PI. parvus, as figured on pi. 3, line a, fig. 6, 20-22, they 

 will be found to be almost identical. 



Var. minutus can by no means be considered the ancestor of var. parvus, on account 

 of closer af&nity of the latter for PL levis, and Hilgendorf has also found it in company 

 with Steinheimensis in the lowest formation. Both Hilgendorf and Sandberger decided 

 that the affinity of this species was closer for PL levis than for var. Steinheimensis, 

 and this is also my own conclusion. My observations agree also with those of Hilgen- 

 dorf, in respect to the derivation of crescens from parvus, and with both his and 

 Sandberger's, in tracing a close affinity with PL minutus. They differ, however, in 

 preferring to trace a direct connection between PL minutus and PL levis, through 

 the normal forms of both species, rather than through the aequiumbilicated varieties 

 of Steinheimensis. 



Again, if we compare a large minutus with the forms of PL parvus having an angular 

 outer whorl, pi. 3, line Jc, fig. 1, this similarity strikes the eye very forcibly. Compare 

 also the figures of parvus on pi. 3, with the those of minutus, line a, pi. 4. This 

 connection with parvus settles the question of size, since this variety of parvus is 

 certainly an intermediate species in this respect, between minutus and PL levis. I 

 have, therefore, separated this angulated form of parvus, figs. 1-4 and 11, line k, pi. 3, 

 under the name of '"i^^'% to distinguish it from the normal forms of parvus, which 

 lead into PL crescens. 



It only remained, therefore, to find some form of a full-grown specimen from Undorf, 

 which would show the characteristics of PL tT- This, on the reception of 

 Sandberger's specimens, was accomplished, and is figured on pi. 9, fig. 16. 



