66 HYATT ON THE TERTIAKY SPECIES 



condition of the young shell is -exactly comparable with the conditions attending senility, 

 as observed first by D'Orbigny among the Ammonites, and subsequently by the author, 

 among these shells, and also in other departments of the animal kingdom. It is apt to 

 mislead the observer, since, although it occurs in the life of the same animal, and in the 

 same organs, it belongs to a class of resemblances which are not generally understood, 

 and have been neglected by all but a few observers. The absorption of, or more exactly 

 speaking the failure of the animal to build up, the costae during the last stages of its 

 existence, causes the whorl to revert to its early smooth condition, and while the latter is 

 due to heredity, the former is evidently pathological in its origin. If a represents the 

 young and its inherited characteristics, and h the new characteristics added during growth 

 to n the adult stage, then a-\-'b-\-n == m, the adult forms. The amount of resemblance 

 between the senile stage and the young, therefore, dejiends i;pon how much or what parts 

 of 6 and n are subtracted by absorption or decay during old age, and as it is never the 

 whole h-\-n, which is destroyed by senile disease, the resemblance produced can never be 

 identical, though they may appear so to the eye in some organs or parts. 



Another way of explaining these phenomena is admirably illustrated by the numerous 

 cases which have been cited of the sudden return of youthfid and apparently long forgot- 

 ten facts, songs, etc., in the memories of old people. They are evidently the survival and 

 the sudden reappearance of youthful characteristics, which have been hidden under a mass 

 of differential characteristics. These being removed the basal form becomes once more 

 visible. 



The foundations of a building are the first to appear, then become invisible under the 

 superstructure, and become visible again only by the decay and destruction of that which 

 they supported. 



SECOND SERIES. 



Planorbis parvus. 



Planorhis Ziefeni (A. Braun) Sandb., Oji. cit. 

 PL m. parvus Hilg., 0/j. cit., fig. 4. 



The shells which represent this variety, pi. 3, line a, figs. 6, 20-22, have a defined upper 

 imibilicus and closely resemble in all essential characteristics those young forms of PI. 

 Stelnhelmensis, which have the mouth deflected downwards and the third carina 

 exceptionally well marked. Figs. 10, 11, line b, pi. 1, represent specimens of this class, 

 which are a trifle stouter than the true parvus and are evidently the young of 

 Steinheimensis, since at earlier periods than the one figured the whorls have all the 

 peculiarities of Steinheimensis. The third carina, which is so prominent in figure 11, does 

 not appear until the shell has attained a stage considerably older than that in which this 

 carination usually makes its appearance in parvus. This is the case also in the more 

 compressed forms of Steinheimensis, such as fig. 9, line m, pi. 1, though in some of these 

 the resemblance of the nearly full grown shells to the young and adult shells of parvus is 

 even closer than in fig. 11, line b, pi. 1. This is undoubtedly attributable to their more 

 disc-like or flattened forms. 



The specimens forwarded by Prof Sandberger resolved the difliculties encountered in 

 the Pit Deposits and explained admirably the close affinities above described between 



