OF PLANORBIS AT STEINHEIM. 61 



Thus, figs. 1-5, 7-8, 10-14, line e, 6-8, line g, pi. 2, show this series perfectly enough, 

 though too slightly magnified to be convincing with regard to the smoothness of the shell. 

 I have been unable to detect any costae on any of these shells, and have seen much more 

 turbinate forms than fig. 8, line g, almost completely uncoiled. I have, however, not yet 

 succeeded in finding the exact equivalent of Hilgendorf 's figure, which must be very 

 rare.-' The forms which have the young so completely trochiform, usually have the 

 last-formed whorls widely uncoiled instead of being so contracted as in his typical 

 specimen. 



On pi. 4 this series is shown fully and may be followed from the minutus forms 

 through such specimens as figs. 6-9, line c, and the more uncoiled forms on line d, 

 to the completely trochiform and uncoiled shells photographed on line e. None of these 

 show any costae. 



The decrease in the breadth of the whorl of the specimens on Imes d and e, 

 as compared with the specimens less uncoiled on line c, and the true minutus forms 

 is also perceptible. The flat form of the young in figs. 10-11, line e, can also be perceived, 

 though less perfectly because the minute size of fig. 11 threw the lower part out 

 of focus. 



This young is like the young of PI. minutus, and of PI. levis, but I failed in 

 finding an adult of any of these forms which could be considered identical with it. 

 This was not a surprising result, since any resemblance occurring at so early a 

 stage with any adult form in the shell alone was not to be anticipated, especially 

 with the adult of any proximate ancestor. 



The law of accelerated development by heredity, which has been noticed in other 

 series, is here also demonstrated. The uncoiling begins at earlier and earlier stages 

 in the different species, and it is the same story with the increase in spirality. If 

 any arrangement in series is in general terms a fair presentation of the natural 

 accession of the forms, then this law must be admitted. It will be found to hold 

 equally well when applied to any serial arrangement of species based on all the 

 attainable evidences of affinity, in the identity of the extreme young stage, the 

 resemblances of the succeeding stages to supposed or traceable ancestral forms, and 

 the similarities of the adult, old age, and diseased forms. 



I have already sufficiently traced the resemblances between these retrogressive 

 imcoiled species and the partly uncoiled, diseased shells of the progressive series photo- 

 graphed. Figs. 4-5, line a, 4-5, line b, and 1-2, 6, line c, pi. 8, are particularly instructive 

 in this connection, since figs. 4-5, line a are senile deformities, figs. 4-5, line b, the results 

 of normal disease, and the remainder doubtful or due to wounds. 



The series traced by Dr. Hilgendorf between this species and costatus, can undoubtedly 

 be formed, but it seems to me perhaps more natural to consider the costate forms 

 as a distinct sub-series. They can certainly be separated quite as easily as any other 

 set of species, if we recognize the fact that the different series all have a tendency 

 to reproduce similar series of forms, which may be arranged in parallel lines. 



1 This defect in my own collection has been most deeply indebted for the beautiful specimen of this form, 

 generously supplied by Dr. Hilgendorf himself, and I feel ■ which he has sent me. 



