92 HYATT ON THE TERTIARY SPECIES 



Formation "m,"New Pit, East side. 



The clay hed in this section was by no means so thick as in Section 8, though it 

 probably only corresponds to the lower part of Formation -'m" in Section 8. The col- 

 lections made from this bed are particularly favorable to the views taken by Hilgendorf 

 of the genesis of PI. oxystomus from PL trochiformis. 



They appear in my collection labelled as transition forms, and would without other 

 evidence be considered sufficient to prove his position. A most perfect series can be 

 built up between PI. trochiformis and the extreme discoidal variety of var. revertens, 

 which I think would be considered sufficient even by the most sceptical person, if no 

 other evidence was forthcoming. 



The confusion arises from the number and variety of the PI. trochiformis with 

 rotundatus young, and the close resemblances of these young forms to certain varieties of 

 Pl.oxystomus, which were also present in considerable numbers. 



PI. crescens was abundant and of large size. 



Formation "x2," New Pit, East side. 



This consisted of the ordinary materials of the shell-sand filling a lenticular pocket, 

 which disappeared on following it a short distance towards Section 8. 



Formation " n, o," New Pit, East side. 



This was a bed of clay of the usual fine textiu^e, and containing specimens of PI. 

 oxystomus and PI. crescens, but no other species. 



Formation "x3," New Pit, East side. 



This consisted of the usual materials, and was apparently continuous with xS, in Section 

 8, and just above it laid the layers of limestone and shell-sand of 



Formation "p," New Pit, East side. 



This bed is directly traceable into formation p, of Section 8, and contained the same 

 species of fossils. 



Little Pit, Section 4. 



Formation " a," Little Pit. 



This was situated considerably to the northward of the Old Pit,- on the first road to the 

 eastward. It was only a small excavation, but I employed a laborer to open it to the 

 depth of about ten feet. The weather, however, was very unfavorable for such work, 

 and the digging difficult on account of the thickness and frequency of the limestone. I 

 therefore abandoned it before reaching the Jura clay, but succeeded in ascertaining the 

 fact that the layers increase in thickness to the nortliAvard. 



Above the two clay layers respectively marked 1 and 2 of Formation a, occurred a 

 thick deposit of drift material, often very coarse, and exactly resembling the porous 

 limestone drift of Section 5, formation a2, but much thicker. Here shells were more 

 abundant, and I obtained a few specimens of PI. tenuis, and one uncoiled specimen of 



