OF PLANORBIS AT STEINHEIM. 7 



Steinheim Period, underlying the sandy strata of the pits, can be approximately proved, 

 and this renders the task of accounting for the origin of the forms in the pits less difficult. 

 It removes the question of the origin from an ancestral form with equal umbilici, a 

 single variety of a species, as figured by Hilgendorf, to that of evolution from several 

 distinct varieties of one species with unequal umbilici, PI. levis Klein, which is, after all, 

 only a form of PL oxystomus ; and also gives a greater allowance of time for the produc- 

 tion of the forms. 



In order to enable the reader to contrast the theoretical views of Dr. Hilgendorf and 

 those given in the first chapter of this essay, and on Plate 9, the following table is 

 appended, copied from Dr. Hilgendorf 's paper, above quoted. 



costatus 



denudatus 



7 

 6 

 5 

 4 

 3 

 2 

 1 



—costatus 



I 



costatus 



I 

 costatus 



crescens 



I 

 crescens 



minutus 



. I 

 minutus 



supremus 



I 

 revertens 



I 



oxystomus 



trochiformis 

 trochiforrais- 



minutus triquetrus discoideus 



-minutus ' discoideus- 



.1 I 



minutus sulcatns 



.1 I . 



minutus tenuis 



parvus Steintieimensis- 

 1 aBquiumbUicatus ^ 



elegans 



rotundatus 



pseudotenuis 



I 

 pseudotenuis 



pseudotenuis 



^ I .. 



Kraussu 



Kraussii 



Steinneimen 

 I 



That Dr. Hilgendorf found these forms in these relations can hardly be doubted ; his 

 reputation and the thoroughness of his] explorations I do not call in question at all, 

 nor do I in any sense doubt that he and others found the PI. Steinheimensis at 

 lower levels than where I found them in my explorations. Our differences rest wholly 

 upon a series of facts which have evidently been treated by two distinct methods 

 of research, and have led to different results. But it will be observed that these 

 differences are not irreconcilable. The fact being that I require more evidence than 

 is found in the Pit Deposits to prove the genesis of the Planorhis trochiformis, out of PI. 

 steinhebnensis^ as far as the succession of the forms in time is concerned, and he has 

 accumulated an immense mass of facts, going to show that the Pit Deposits are aU sufficient, 

 and contain the whole history of the series. This question, and others connected with it, 

 are discussed in the following pages, and it only remains for me to express my earnest 

 thanks to Dr. Hilgendorf for a valuable series of his type specimens and for his courteous 

 frankness in sending me accounts of his work while in progress, and to regret that 

 he has not yet published his last researches. Dr. . Sandberger has also treated me with 

 great kindness, and I have to thank him for material assistance, especially for specimens 



