1905.] NATURAL SCIENCES OF PHILADELPHIA. 359 



tnbercled shoulder keel. On the other hand, it has introduced a senUe 

 character which occurs normally in a near relative (if not an actual 

 descendant) of Fulgur canaliculatum, namely, Fulgur pijrum}^ 



Summary. 



In most Mesozoic and Cenozoic gastropods, changes in ornamentation 

 occur in going from the earlier to the later whorls. A normal succession 

 of developmental changes may be observed, which varies little in 

 widely separated groups; but most families show certain peculiarities 

 in the succession which distinguish them from other families . Infancy , 

 youth and maturity are represented by their peculiar styles of orna- 

 mentation in the ontogeny of an individual ; but these stages, with their 

 characteristic styles of sculpture, cannot always be correlated with the 

 adults of a previous geologic time. This latter discrepancy may be 

 due, in part, to the imperfection of the geologic record ; but the writer 

 inclines to the behef that the unequal acceleration of characters plays 

 an important part, and that features which occurred at the same time 

 in an ancestor are apt to be widely separated in the ontogeny of a 

 descendant." 



In the last whorl, or in the last few whorls, of many gastropods of dif- 

 ferent groups we find the characters of senility. These characters 

 or marks of senility do not necessarily appear together in the ontogeny 

 at exactly the same time. Some occur far in advance of others; that 

 is to say, there is frequently an unequal acceleration of the senile 

 characters. 



These senile features, several of which are, as a rule, found together, 

 are the last characters which occur on the shell, for their appearance 

 is the forerunner of the death of the individual . As might be expected , 

 senile species or genera of fossil gastropods never transmit descendants 

 to later geologic formations, but represent the end members of short 

 branches on the phylogenetic tree. 



The writer believes from his researches that the forces of evolution 

 sometimes work rapidly and sometimes work slowly, and that the 

 forms in which the evolutional rate is rapid are bizarre, senile offshoots. 

 On the other hand, forms which have a long time range go t>hrough a 

 slow evolution, though the developmental features and even the order 



•3 There is good reason for believing that F. pyrum and its numerous varietie^ 

 ■ire descended from a Miocene form which is either identical with or closely aUied 

 ?; F canal?c./ahm. See Smith, Burnett, Proc^^Acad^ Nat Sci PkUa June, 

 1902 and Grabau, A. W., Am. Nat, August, 1903, p 521, and tab e, p. o39. 



i< Smith James Perrin, "The Development and Phylogeny of Placenticeras, 

 Proc. Calijornia Academy of Sciences, Third Series, Vol. I, No. 7, p. 185. 



