JAMES PERRIN SMITH 15 



tendency in all the Trcpitoidea to form keels late in the history of the 

 stock. All true Tropitoidea show this character well developed, and 

 among the Ilaloritidaj Homerites shows the same tendency, and develops 

 a vestigial keel just hefore maturity, losing it entirely at maturity, as 

 shown on PI. VI, figs. 16-21. Leconteia H. and S. has a somewhat similar 

 history, with the same retardation of the septa, and the same reversion to 

 the Pericylus ornamentation, but without the formation of a keel at any 

 stage. This form is also illustrated on PL VI, figs. 11-15, for comparison 

 with Metasibiritcs and Homerites. This parallel development in ratlier 

 closely related genera may be called orthogenesis. The reappearance of 

 the Pericydus and Sibirites characters is undoubtedly atavism, but the 

 parallel development of the keel in Metasibirites and Homerites can only 

 be ascribed to the bringing out of a tendency always present, though 

 previously latent. The immediate cause, in both cases, is the disturbance 

 of heredity consequent upon arrest of development and incipient degen- 

 eration. 



Another clear case of arrest of development is seen in Clionites, of 

 the Upper Triassic. This genus, when fully developed, has the sculpture 

 of Trachyceras, and a form something like it, but the septa are ceratitic ; 

 and even when nearly mature Clionites is evolute and sriuare shouldered, 

 with prominent shoulder knots, like Tirolites, of the Lower Triassic. This 

 genus, then, has a mixture of characters that ought not to occur together. 

 In the ceratitic septa it shows a stoppage of development in the stage 

 characteristic of Middle Triassic forms, and in the square-shouldered 

 whorl and shoulder knots it has been arrested in a stage corresponding to 

 a Lower Triassic genus. But in its sculpture and in the median furrow 

 it is as far along as its immediate ancestor. This is shown on PL XV, figs. 

 1-8, in the development of Clionites {Traskites) robustus, where the 

 youthful stages are very like Tirolites, difi'ering from it in the possession 

 of the median furrow, inherited from an ancestor later than Tirolites. 

 The mature stage of Clionites takes on the sculpture of Trachyceras and 

 approaches it in form, but fails to reach the complexity of septation of 

 that genus. Ordinarily Trachyceras does not show any trace of the Tiro- 

 lites stage, but in the lower part of the Upper Triassic there are several 

 species which have prolonged their ontogeny, and do show a distinct 

 Tirolites stage. Such a species, Trachyceras duplex, \i figured on PL XV, 

 figs. 13-16. This species shows the beginning of retardation, which is 

 more complete in Clionites. Still more complete arrest of development is 

 seen in Clionites {Calif omit es) Merriami, PL XV, figs. 9-12, which has re- 



