No. 381.] A HALF-CENTURY OF EVOLUTION. 665 
uncoiling of the ammonites into forms like Scaphites, Crioceras, 
Helioceras, Turrilites, and Baculites, were originally perhaps 
distortions due to physical causes somewhat similar to those 
which produced a loosening or uncoiling of the spire in Pla- 
norbis. These variations or distortions of the pond snail, signs 
of weakness, the result either of pathological conditions or of 
senility, were due to unfavorable changes in the environment, 
such as either a freshening of the water or some other chemical 
alteration in the relative amount of alkalines and salts. The 
changes in the ammonites, though more remarkable, are simi- 
lar to the aberrations observable in the shells of the upper 
and later layers of the Steinheim deposits, made known to us 
by Hilgendorf, Sandberger, and more especially by the detailed 
and masterly researches of Professor Hyatt. 
In this case the Miocene Tertiary Planorbis levis was sup- 
posed to have been carried into a new lake, before untenanted 
by these shells. Although from some unknown cause the lake 
was unfavorable to the production of normal /evis, whose 
descendants show the results of accidents and disease, yet, 
owing to isolation, which prevented intercrossing with the 
present stock, and to the freedom from competition, the species 
was very prolific, and the lake became stocked with a multitude 
of more or less aberrant forms constituting new species. Some ~ 
of them are nearly normal, with a flat spire, others are trochi- 
form, and others entirely unwound or corkscrew-shaped. Simi- 
lar aberrations occur in Planorbis complanatus, living in certain 
ponds in Belgium (Magnon) ; in the slightly twisted planorbid 
Helisoma pexata Ingersoll of St. Mary’s Lake, Antelope Park, 
Colorado, and in the unwound forms of Valvata first found by 
Hartt in Lawlor’s Lake near St. John, New Brunswick, and 
described by Hyatt.1 In all these cases of parallelism or con- 
vergence the aberrations seem to have been due to some 
unusual condition of the water advèrse to normal growth. 
Hence, it is not impossible that the singular uncoiled or 
sively dependent for existence upon each other.” He also suggests that the 
development of the cuttles “has been commensurate with that of thecetacean 
order, of some of which they form the food.” (Phil. Mag., vol. xxiii, 1862, p. 384.) 
1 Ann. Rep. Hayden’s U. S. Geol. and Geog. Survey Territories. 
