58 HYATT ON THE TERTIARY SPECIES 



TV. Descriptions of Series and Sub-Series. 



FIRST SERIES. 



Planorbis minutus. 



Planorhis muUifortnis minutus Hilg., Monatsber. d. Akad. d. Wissensch., Berlin, July, 

 1866, f. 15. 



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



PL hemistoma Klein, Jahresliefte 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 affinity for PI. crescens. It differs, 

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

 resemble PI. Krmissii, 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. ininuhis, 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. pta-rvus, as figured on pi. 3, line a, fig. 6, 20-22, they 

 will be found to be almo.st identical. 



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

 of closer affinity of the latter for PI. 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 PI. 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 p>arvus, and with both his and 

 Sandberger's, in tracing a close affinity with PI. mimitus. They differ, however, in 

 preferring to trace a direct connection between PI. minutus and PI. 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 PI. jKirvus having an angular 

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

 also the figures of 2^(i'>'vus on pi. 3, with the those of mimitus, line a, pi. 4. This 

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

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

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

 under the name of "1"w"% to distinguish it from the normal forms of parvus, which 

 lead into PI. crescens. 



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

 which would show the characteristics of PI. "S". This, on the reception of 

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



