No. 490] 



ORTHOGENETIC VARIATION 



611 



in mind that a (liU'erence may here he found between protoconcli 

 and concli. When tlie whorls embrace but sh'glitly, a dee|) suture 

 is produced between them. Jn this case (lie spire also is a slender 



fig. 6), As the amount of eml)ra( in<: iiiciva-c-^. ilic >iitiin' becomes 

 less strongly de|)ressed, and the s|»irc take- on an vwv ^i-raicr apical 

 angle. The amount of eiubracitiLi" niay incicaM- until suture 

 is found at the and.iin. of ilic piwdini:' u hoH Mndir. 1 1 , (ig. b3). 

 Ikvond this, the cmbrarinu- is only < am.Ml in ilu" old a-c of the 

 individual of normal types or in spcciaii/cd types, generally the 

 members of a phylogerontic series.^ 



In a large number of types, the amount of embracing by the 

 whorl remains practically the same throtighout life, thus giving 

 the spire a uniform angle. In others, again, and perhaps in the 

 majority of specialized types, the embracing is at first less, but 

 slowly increases in amomit with each succeeding whorl in the later 

 stages (Fig. 1 ). 



Even in dcgradational tyj)es, where the embracing of the adult 

 shell is in extreme excess of that of the normal adult type, the 

 amount of embracing incieases regularly from its first appearance 

 to the comi)letion of the growth. 



There is another extreme found in phylogerontic members of a 

 certain group of Gastropoda and so far observed in the non-<;>ana- 

 liculate types only; namely, the loss of the power to coil, due to the 

 e(|ualization of the rate of increase in all portions of the shell 

 (Studies I, figs. 14 and 15). This results in a looser coiling or 

 complete straightness of the final jwrtion of the last %vhorl, and 

 may or may not be accom{)lished l)y an increase in the diameter 

 of the whorl. Ontogenetically this is often expressed by a progres- 

 sive loosening of the coil, though there are various degrees of 

 abruptness, some coils becoming gradually straightened, while in 

 other types this straiu'lilenini^ apin^ars very abru])tly. As will be 



ii,(tinc youtlihil ..r ;i(l.ilcs( (-iit ; cphebic, adult; gerontic, old a^e or senile. 

 (\)nv>i><iii(liiii,r st.iiics 111 phyloircnesis are designated by the prefix phijlo, but 

 the tenu pInjUxjn-ontic. or nu i;d old age, is the only one in common use. 



