76 HYATT ON THE TERTIARY SPECIES 



The umbilicus on the lower side is narrow, and it will be observed that the young are 

 quite stout in some specimens, as in figs. 11, 12, line d, pi. 2. Since the above was 

 written, I have succeeded in finding in my collection true PI. stefuMimLis, in fiarmation a, of 

 the Old Pit, line/, figs. 1-3, pi. 1. 



The prominence of the third carination in PI. Kraussii is precisely what was 

 previously observed in PI. Steinheimensis, and besides this the adults of many 

 develop the second carina and become flattened externally on the last whorl, as in 

 the young of PL tenuis. Occasionally, also, as in fig. 2, line d, pi. 2, a faint sulcation 

 becomes visible on the upper side. 



It is possible that the specimens figured on pi. 2, from the higher formations of 

 the Pits, are the survivors of the forms of formation a, and perhaps may be considered ks 

 somewhat dwarfed. Their resemblance to the specimen from formation I, figured on pi. 

 2, line q, fig. 12, as PI. stJSmensis, is apparent at a glance. 



The specimens of Kraussii figured on line d, pi. 2, fig's. 13-16 are similar to some of 

 the more compressed young of PI. tenuis, like those of figs. 7, 13, line 7i, pi. 1, and figs. 

 1-2, line f, pi. 7, and are identical with the more immature young, and some of the 

 full grown shells of PL tenuis of the Cloister Ridge rocks. Sandberger appears to 

 have been unable to separate this form in the Pits from PL tenuis, but there is no diffi- 

 cvilty in doing this until the young of the tenuis forms are studied. The adults are quite 

 distinct. Hilgendorf's opinion that this species is a direct derivative from PL Steiiiheim- 

 ensis is amply sustained by the material I have examined, but whether this ought 

 also to be considered intermediate between Steinheimensis and his PL pseudotenuis is more 

 doubtful. This last I have had no means of studying, except through two specimens sent 

 me by Hilgendorf. These are very minute, delicate, thin-shelled specimens, with the third 

 carina only developed, which forms a sharp ridge on the lower side and outer edge of the 

 whorl. The shell has the aspect of the young of PI. tenuis, but I have unfortunately not 

 the time now for a re-examination in order to test this question. If his material 

 enables him to trace a close series of transmutations this can be established, but in the 

 absence of this exact proof, I should certainly at once class it as the yovmg of a variety 

 of PL tenuis, traceable to such forms of PL sieiS^sis ^s are figured on pi. 1, line c, 

 fig. 14. 



There is not a single specimen of PL pseudotenuis figured on my plates, and this 

 shows the extreme rarity of the shell, a fact I was not aware of until the receipt of 

 Dr. Hilgendorf's type specimens. If this series is finally established by Dr. Hilgendorf, 

 then another distinct series of a retrograde character will have to be added to those des- 

 cribed in these pages. The specimens of PL m. Kraussii received from him are of the 

 true steMehZ.sis type, and show none of the tenuis-like characteristics here described as 

 varieties of this species. Fig. 9, line/, pi. 2, is identical with these. 



Variety tenuis. 

 The sub-varieties appear to be almost wholly derived from forms of the precedinc; de- 

 cribed varieties, either of Steinheimends or suMmcnm- Thus the first sub-variety which 

 shows a tendency to become turbinate, such as that figured on line c, pi. 1, fio-s. 2-12, 

 appears to be connected directly with the Steinheimensis forms, similar to fig. 16, line h, 

 previously described. 



