OF PLANORBIS AT STEINHEIM. 47 



I am, of course, bound to believe Dr. Hilgendorfs explicit statement that he was 

 obliged in order to reach the Jura, to dig much deeper than I did, and there are certain 

 facts in my own sections which confirm this. He dug to the Jura in his Sections 1 and 2, 

 before the Pits were opened so far into the side of the hill, and was therefore farther out 

 in the valley, and this corellates with the facts in my Sections 5 and 4, as compared with 

 6. In 6 the lowest formation a is really equivalent to a, 3 in No. 5, which was only a 

 short distance to the northward, and is replaced by a layer of limestone in No. 4, which 

 was a considerable distance to the north and east. This is shown by the position of the 

 limestone immediately between the clays above, and the coarse layer below, in No. 4. 



Thus all the layers found below a, 3 in No. 5, and the limestone just above a, 3 in 

 No. 4, are absent in No. 6. The first of these is the coarse layer a, 2, in Section 

 5, and a, 3, in Section 4. The great increase in the thickness of this, is carried out in 

 the Little Pit in the other beds also, so that it can safely be inferred that the beds grow 

 deeper outwards in this direction, as well as probably to the westward where Dr. Hilgen- 

 dorf dug. Thus a, 1, in Section 5, is shell sand and clay, becoming two layers and pos- 

 sibly more of clay, with at least three thick limestone partings in No. 4. 



The conditions governing the deposition of the layers between these three places, 

 therefore, must have been very different, though going forward at the same time, and at 

 a short distance from each other, and two of them, 5, 6, in nearly the same depth of water. 

 This conclusion is sustained also by the aspect of the layers containing fish remains, which 

 occur in h, c, d, according to the observations made upon the adjacent Sections 3, 4, r5, 

 and 6. The pit formations, in fact, show everywhere the exceeding variability of the con- 

 ditions under which deposits took place in adjacent spots, for they cannot be dignified by 

 the name of localities. 



The fossils found in a of Section 6, comprised nearly the whole range of forms, 

 and indicated also by the worn character of some of the trochiformis shells that they had 

 colonized this locality from some other part of the lake. Those found in the lower part 

 of a in contact with the Jura contained eight specimens of PI. trocldformis, all in one 

 spot together, with intermediate varieties Sw/e™'% while at another place in the hole, only 

 three feet distant, none were found of this species. No divisions were seen in this stra- 

 tum, but the specimens were fewer in the upper part than in the lower. Pish remains 

 occurred in a soft calcareous parting immediately between this and formation h. 



This last fact is important, since it confirms the opinion that in all probability a iu 

 Section 6 is equivalent to a, 3, in Section 5. 



Do the fossils differ in Section 5 ? The list given shows that in formation a, 3, only the 

 following are found, PI. Steinheimensis, tenuis, discoideus. If I had gone no farther, the 

 absence of PI. trochiformis, and the intermediate forms PL ^''aiJ^^Zi' might have been 

 considered very significant in favor of Dr. Hilgendorfs view. 



But the next lower formation a, 2, Section 5, at a lower level, contained the same gen- 

 eral association of forms with the addition of PI. discoideus, and 'SfS"- The lowest 

 formation a, 1, contained even a more complete association of forms, and was not separable 

 from a in Section 6, with which the specimens were finally mingled on plate 1 in 

 order to complete the illustration of this stratum, which in both cases rested on Jura 

 clay. But there was a much greater abundance of PI. sulcatus, and the PI. trochiformis 

 was exceedingly rare, only one specimen being found, and that a rolled shell. 



