82 HYATT ON THE TERTIARY SPECIES 



discoideus. The extremely flat and square whorled varieties with their prominent 

 carinae and sulcations being, however, more rarely represented by true trochiform 

 descendants, than those forms of discoideus in which these parts are less prominent, and 

 the form from the beginning of the series more asymmetrical. These in the trochiform 

 descendants appear in the young as the rotundatus-like form, by which the greater part of 

 all the trochiform specimens are characterized. Thus the direct line of descent from PI. 

 Steinheimensis is shown to be prepotent, or to have a greater number of descendants 

 than any other form. 



It is useless to attempt to reduce these proportions to numbers, but it may be generally 

 stated that in any formation the trochiform varieties with rotundatus-like young greatly 

 outnumber all others ; so much so that those with a distinct flattened form of the apex 

 have to be sought for, and become rarer and rarer until the extreme is reached in 

 sulcatus-like young of elegans. 



The value of this statement has been tested by the revision of boxes of PI. trocMformis 

 from nearly all the formations, the results of the sifting of several samples from each for- 

 mation in the New Pit, besides a mixed box containing thousands, and a similar review 

 of material in the Old Pit and East Pit. There is, however, one notable distinction which 

 is represented also in a measure in the plates. The forms with rotundatus-like young 

 occur much more abundantly in the East Pit than in the Old or New Pit, where the forms 

 with discoideus-like young, the PI. trochiformis, as figured by Hilgendorf, appear to be 

 equally, if not more abundant in some formations. 



It will, however, be noted even in this connection that all of the rotundatus-like young 

 as well as the larger forms with rotundatus-like young figured on plate 2, came from the 

 Old Pit. The cases of distorted spirals also are miich more frequent among the specimens 

 with flattened discoidean apices than in those with normally formed or rotundatus-like 

 apices. This fact correllates with another of equal importance, namely, that in some 

 case a true elatior-like whorl may be caused by a wound. Thus in one specimen in my 

 collection, of a PI. trochiformis with a typical rotundatus-like young, very smooth and 

 rounded on the apex, a wound has interrupted the deposition of the shell when nearly 

 half grown. The new whorl, when growth was resumed, was not only begun considerably 

 inside of the spiral projected by the earlier whorls, thus narrowing the spiral consid- 

 erably, but a first carination was produced, and a form of the whorl similar to that of 

 elatior instead of such a form as in plate 2, line s, fig. 6, which would have been produced 

 in due time if no wound had taken place. 



These facts and others mentioned previously, are very curious and appear to indicate 

 less strength on the part of the prominently carinated forms to resist external or internal 

 causes of disease, producing a lessened size of the whorl and distortions of the spiral, 

 which are only very rarely found in those forms having the rotundatus-Hke young. Thus 

 the descendants in the direct line of descent from Steinheimensis are not only more 

 numerous, but healthier than those of other varieties. The largest specimens are those of 

 the transition forms from the flat variety of discoideus to PI. trocMformis, such as are 

 figured on line m, plate 2, and others with the rotundatus-like young. 



The descendants of the sidcatus and elatior varieties are generally smaller, especially 

 when distorted, but when perfectly normal may be quite up to the average size of 



