322 GEOLOGICAL BIOLOGY. 



examining and trying a number of hypotheses to account for 

 these facts, the following definition seems to be fairly satis- 

 factory : TJic species in its specific characters shows a greater 

 degree of variability or plasticity in the earlier than in the later 

 stages of its history. 



Atrypa was an illustration of the remarkable continuance 

 of the stage of plasticity, but we observe that the particular 

 limitation of range of the plasticity became thereafter a 

 specific characteristic of the race. The greatest and the 

 least number of plications attained by any representative of 

 the genus are probably met with within what has been called, 

 in a broad sense, the species Atrypa reticularis. Another 

 law of specific modification is seen in the gradual jiarrozving 

 of the limits of the plasticity — one series concentrating about 

 the forms with few plications, the other series concentrating 

 about the forms with many; — the one expressing the law of 

 retardation of growth for this character, the other the law of 

 acceleration for the same character. 



The Initiation of the Species of Ptychopteria. — Ptychopteria* 

 is a remarkable instance of variability among the initial rep- 

 resentatives of a genus. The case is as follows : A genus of 

 Lamellibranchs, having some well-defined generic characters, 

 is first seen in the upper sandstones of the Neodevonian in 

 Western New York and Pennsylvania. A few years ago 

 the genus Ptychopteria was defined and figured by the 

 New York State Geologist, f and nearly a score of species 

 were described from different localities and, possibly, different 

 geological horizons. About the time of the publication of 

 the species a block of sandstone, about a cubic foot in size, 

 was found in Chautauqua County, fallen from a ledge of the 

 Panama sandstone, containing many hundreds of specimens 

 of shells of this genus. These were carefully collected, 

 sorted, and classified according to the characters by which the 

 several species defined by Hall had been distinguished. An 

 analysis of the species already described showed the following 



* The facts of the case were briefly alluded to in a paper "On Devonian 

 Lamellibranchiates and Species making," — referring to species which paleon- 

 tologists make, and not to the origin of species. Am. Jour. Sci., vol. xxxil,. 

 p. 196. 



f Paleontology, New York, vol. v, " Lamellibranchiata." 



