1895. ] NEW YORK ACADEMY OF SCIENCES. 69 
This curvature of the shell of Crepidula is an unsymmetric 
one. The radius of the curve is very much shorter on the right 
side than on the left. 
Secondly, the direction of the growth is a different one in the 
two shells. In Pecten the shell grows along a straight line from 
A to B, whereas in Crepidula the shell grows in a curved line 
(the beginning of a spiral) from a to 8. 
Those two facts (the asymmetry and the spiral growth) explain 
why the ridges on the surface must be curved very slightly. 
I report this case at some length because it seems to me that 
this may give us some light as to how a new species may be 
created. 
In Crepidula we see a form that begins to adapt itself to an 
ectoparasitic mode of life. It is no parasite yet, but clings to 
the surface of other animals, and thus profits by the small par- 
ticles of food that are wasted by the latter. 
Crepidula still has a certain power of motion, which it will 
entirely lose with time. 
It is evident that it will be favorable to the species if the 
individuals cling to those animals that have a rapid motion, as 
they will in consequence be carried through the water and be in 
contact with a continually changing medium, fresh oxygen, ete. 
We know that Pecten is a lamellibranchiate mollusc endowed 
with a strong power of motion. 
Whoever has watched scores of scallops dancing in the water 
will remember it as an exceedingly pretty sight. The mollusc 
moves by rapidly opening and closing its valves, pushing out 
water with great vehemence, and thus progressing in backward 
jumps. As the water is pushed out with more strength on one 
side of the animal than on the other, a motion results which is 
only comparable to a sort of waltz. This motion is very quick 
and kept on for a very long time. 
It is clear that it will be of more use to Crepidula to be 
attaehed to a quickly moving Pecten than to a slowly creeping 
lobster, or an immobile oyster. Thus we might expect that by 
natural selection those individuals that are attached to Pecten 
would have more chances to survive. As bye and bye the power 
of motion will be lost for Crepidula, the surface of the animal, 
which is attached to Pecten will present ridges, and thus we may 
in time obtain a ridged Crepidula instead of a smooth one. 
Everybody will agree that regular ridges on the surface of a 
shell constitute an important species character, and out of these 
facts we may conclude that we shall get a new species in the 
ridged Crepidula. 
DEPARTMENT OF BroLoey, 
Co.tuMBiIa COLLEGE, NEw YORK. 
