ATAMSM OR REVERSION 975 



reversion of the species to ancestral characters may often be seen 

 to be nothing more than an arrestation of development at an imma- 

 ture morphic stage, when the characters of the young are like those 

 of the adult ancestor of earlier geologic horizons. This arrestation 

 of development or genepistasis is only a step removed from pro- 

 gressive retardation in development of which it forms the distal 

 limit. In either case, whether complete or partial arrestation ( heter- 

 epistasis) occurs, or whether development is retarded altogether 

 or along certain lines, the resultant form will seem out of place 

 in the horizon characterized by the more advanced species, and may 

 sometimes lead to mistaken classification of the strata containing 

 them. 



As an example of atavistic individuals occurring in a horizon 

 above their normal may be cited the case of the Devonic brachiopod 

 Spirifer mucronatus and its derivatives already mentioned. This 

 species, so far as known, is represented by the elongated, strongly 

 mucronate type in the lower Hamilton beds of eastern North 

 America. In the Upper Hamilton of Ontario occurs the mutation 

 S. thedfordense, which in its younger stages has all the features of 

 the typical lower Hamilton form, but in the adult it is proportionally 

 much less extended, without mucronate points, without the charac- 

 teristic plication in the median sinus of the pedicle and groove in 

 the median fold of the brachial valve, and with strongly marked, 

 regular lines of growth. With this species occur a number of 

 individuals which have been retarded in their development and 

 which, hence, recall the ancestral type in some of their characters — 

 notably the strong mucronate lateral extensions. In the New York 

 province of the Middle Devonic the chief developmental changes 

 acquired by S. viucronatus consist in the relative shortening of the 

 mucronate angles as the shell approaches the adult stage, until 

 from a width several times its height it had changed to a form in 

 which the width was more nearly equal to the height, and in which 

 the mucronations were wholly obliterated. In other respects 

 the changes were very slight. With this mutation occurs not infre- 

 quently a form showing arrestation of development (genepistasis) 

 at an early stage, and hence it retains into the adult its youthful 

 mucronate' character, which recapitulates the ancestral condition. 

 These arrested individuals thus recall in practically all respects 

 the primitive mucronate type from which they are derived. 



An example of a retarded individual from the Karnic lime- 

 stone of the California Trias is cited by J. P. Smith. This was 

 an immature specimen of the ammonoid genus Trachyceras which 

 had persisted unusually long in the ancestral Tirolites stage, thus 



