NEW YORK ACADEMY OF SCIENCES 575 



the evolution of some Devonic spirifiers. Spirifier mucronatus 

 (Conrad) is a Linnaean species comprising a large number of 

 mutations. A remarkable fact is that all mutations pass 

 through a mucronate type such as is characteristic of the typical 

 mutation after which the species is named. (The term muta- 

 tion is here used in the sense in which it was originally proposed 

 by Waagen, and not in that in which it was subsequently used 

 by De Vries; i.e., for the result and not for the process.) A 

 still earlier stage in development (nepionic) shows the non- 

 mucronate features of the ancestral species similar to S. duo- 

 denarius of the Onondaga. The mucronate feature is carried 

 to excess in a number of mutations of the Lower Hamilton 

 group. It is especially persistent in the Michigan region. This 

 type of outline is accompanied by a rib in the median sinus and 

 a depression in the fold. In Ontario the primitive mucronate 

 type gives rise upward to a number of mutations which are 

 especially characterized by progressive increase in height with- 

 out corresponding lengthening of the hinge. The median 

 plication and depression quickly disappear. 



Acceleration and retardation in development are the chief 

 principles which explain the development of the great number 

 of mutations. For the principle of retardation the term brady- 

 genesis (from fipadvz, slow,) was proposed, corresponding to the 

 term tachygenesis proposed by Hyatt for acceleration. 



In the New York province the primitive mucronate type 

 gives rise to high and short-hinged mutations, but these retain 

 the median rib and depression. In form these are tachygenetic ; 

 in respect to the surface features, bradygenetic. In the are- 

 naceous beds of the later Hamilton in eastern New York, a 

 mutation with many ribs and moderate mucronations exists. 

 This is in many respects a bradygenetic type. 



Side by side with extremely accelerated or tachygenetic types 

 in all horizons (i.e., very short-hinged, non-mucronate, high and 

 thick mutations) occur slightly retarded or bradygenetic types, 

 which retain in the adult the mucronate character which is 

 typical of the young of all the mutations. 



A. W. Grabau, 



Secretary. 



