PEESIDElSri's ADDEESS. 281 



tribe or species in the adult (aggregation al development), will in their 

 old age lose their ornamentation by stages that reverse the order in 

 which it developed and finally revert to a smooth-whorled condition 

 (degradational development), just as Hyatt found to be the case in 

 Ammonites. 



All these successive stages follow the sequence outlined above in 

 regular order, and to this Grabau applies the term " Orthogenetic 

 Variation." To quote his own words: "Orthogenetic Variation may 

 be defined as progressive variation along definite or determinate 

 lines, whether such variation is along the line of increasing or 

 decreasing complexity, i.e. aggregational or degradational " [S8, 

 p. 607). 



The law of tachy genesis is well shown in Gastropoda when a series 

 of forms are traced through successive geological epochs, as Grabau has 

 done for Ftilgur, Sycotypus {27), and Fusus {29). 



That the study of orthogenetic development in the different families 

 of Mollusca, and more careful attention to the character of the 

 protoconch,' due allowance being made for differences in individuals 

 living under diverse conditions (the Heterostylism of Boettger (7)), 

 may lead to the sorting out of forms, hitherto classed together ov?ing 

 to similarity of form in the adult, is more than probable. It may also 

 lead to the phylogenetic association of groups hitherto considered to be 

 unrelated.^ It is not, however, possible to follow Grabau when, led 

 away by enthusiasm for his special study, he maintains that similarity 

 of lingual dentition should be subordinate to shell characters {27, 

 p. 537). Nor can assent be given when he classes patelloid shells 

 with uncoiled and, according to him, degenerate forms like those of 

 Vermetus, Cyclosuriis, and many another from Lower Cambrian times 

 to the present day {26, pp. S38-9 ; 28, p. 623). The patelloid shell 

 is not the result of uncoiling, but of a total alteration in growth to 

 suit it to the animal's mode of living. To label Patella and other 

 cognate forms pbylogerontic is to misajiply that much abused and over- 

 worked term. One might equally describe Denialium as pbylogerontic I 



In this matter it would seem as if the disciple had, as disciples 

 are apt to do, gone further than the master, for Hyatt points out 

 {1^2, p. 588) : " There is an obvious correlation between coiling of the 

 shell and the habit of crawling. Thus all univalve crawling mollusca 

 have this general tendency. Among Gasteropoda this is well known, 

 and those shells which degenerate and tend to lose the spiral mode of 

 growth and become irregularly straightened out in these older stages 

 of growth, are forms which become attached or lead sedentary lives, 

 i.e. Vermetus attached late in life and Magilus buried in coral. The 

 most significant case, however, is that of Fusurella, which has a coiled 



' "I believe it is not too much to say that the protoconchs of all the species withiu 

 a given genus should agree as to their essential characteristics, and that no 

 species can be considered congeneric in which the protoconchs show a radical 

 difference" (Grabau, 26, p. 922). 



- Grabau puts Buccimim and l<'ulgur w'lVn Fasciolaria, and 3Teloiit/e>ia and Heinifusus 

 with I'nsus (27, p. 537), considers Levifi(sus closely related to I'leurotoma (27, 

 p. 526), and hints that Pterocera is polyphyletic [26, p. 930, note). 



