Xo. 490] 



ORTHOGENETIC VA RIA TION 



635 



condition is observable in Turritella, where the early species are 

 generally more loosely coiled. Such looser coiling is also observable 

 in the young of modern forms, in the adult of which the whorls 

 embrace up to the angulation. Here however another factor enters 

 in, the progressive flattening of the whorls so that the spire remains 

 slender even though the whorls embrace to the ambitus. Similar 

 conditions obtain in Nerinea, Cerithium, and others, the surface 

 of the whorls even becoming concave in many of tliese. 



In its most pronounced form the progressive increase in the 

 amount of emln-acing of the w^horls is seen in j)liyl()^er()ntic tvj)es. 

 Here this increase is accompanied by a loss of ornamentation and 



ingis seen in Melono'ena, wliere the earli(>r whorhs become to some 

 extent enwrapped by tlie later ones, the form of these later whorls 

 bearing no relation to that of the earlier ones, but being without 

 the normal ornamentation of the earlier whorls (Studies I, fig. 9). 

 Similar though more regular conditions are found in the clavili- 

 thoids for a discussion of which the reader is referred to my "Phy- 

 logeny of Fusus." Cossmann has recently reiterattMl hi> belief in 

 the generic relationship of ClaviHtluN and ('yriiilus which latter 

 type is a phylogerontic terniinal of the ino(l<Tn I-"n>ns scries. 'J^iis 

 reassertion of his former position in,licatc> that Cossmann has 

 either not carefnily read my arunment.s for the total <listinctness 



them as valid. If this is tlu- < ase I nmsi ^ive np all hope of con- 

 vincing him, for 1 do not see that I can state the case more fully. 



