OF PLANORBIS AT STEINHEIM. 49 



very carefully, by digging into the sides of the Pit, for the few specimens of PI. tenuis 

 which accompanied it.^ 



But what is more important, and to me most inexplicable in this matter, is that 

 I failed to distinguish throughout any beds which could be considered as corresponding to 

 those described by Dr. Hilgendorf as sulcahcs, ''slftJaufy discoideus, 'X'cilS's') trochiformis 

 and oxystomus. 



There were here and there beds, such as a 3, Section 4, which held only PL tenuis, 

 but this had also "ZlXT ; « 2, Sect. 5, could have been 'XSSf , but here was also 

 trochiformis ; and a 3, Sect. 5, could also have, but for this, been considered a true 

 Discoidetis bed. 



PI. minutus came information d, Old Pit — but how account for the earlier appearance 

 of PI. ''^ufuluf in a 3, Sect. 4, so much earlier, before the fish layers, and in what would 

 otherwise have been a perfect PL tenuis bed. In the New Pit the same difficulty occurs 

 with PL oxystomus and crescens which, as described above, put in an appearance too 

 early, and spoiled the definition of the PL trochiformis formations. 



So also, in the East Pit, formation d, e, otherwise a perfect PL discoideus formation, 

 with PL triqueirus, and PL costatus and mimitus as described by Dr. Hilgendorf, con- 

 tained one or two broken specimens of PL oxystomus. Not much in themselves, but 

 very significant when taken in connection with other facts, and also when it is considered, 

 that to obtain these, I sifted considerable material taken from this very thick formation. 



In this Pit, however, I could easily trace the kind of evidence brought forward by 

 Dr. Hilgendorf, and from d, e to the true Trochformis bed /*, which would give a very 

 perfect series from PL discoideus or sulcatus to PL trochiformis. 



I can think of no way to account for these discrepancies, except the different results 

 of different methods of research. My collections are much smaller, and my observations 

 more limited than those of Dr. Hilgendorf, and therefore it may seem to some readers 

 that it is presumptuous on my part to oppose his results, but from another point of view 

 this only makes it more difficult to account for the exceptions which were found. 



I feel myself, that the conclusions with regard to the Cloister Ridge rocks need more 

 positive evidence than I have been able to bring forward, but the facts with regard to the 

 occurrence of fossils in the Pit Deposits, are not in any case theoretical. The identifica- 

 tions are made after comparison with a set of types sent me by Dr. Hilgendorf, and after 

 prolonged and repeated observations, and the discrepancies occur in species of marked 

 characteristics, and easily identified. The method of research excludes error in any other 

 respect as much as is possible in such investigations. 



The theory also, which I have advanced, that the Cloister Eidge rocks really contained 

 a more ancient fauna than the Pit Deposits, is substantiated by the geological facts, 

 the fauna contained in them, the " tenuis Tufa," found lying on the Jura Clay, by Dr. 

 Hilgendorf, and described by him in a letter to me, the structure of the lower beds of the 

 Pit Deposits, and so on. 



Again, the situation of the Cloister Pit, where Dr. Hilgendorf found a full series of beds 

 near the highest elevation of the ridge, and which must be at least a hundred feet 



1 The sides of the Pit above in which the PI. denudatw' this shell difTers in color from them, and agrees with the 

 also occurs in formation e were removed well back, and bleached, dead white PL tenuis found with it. 



