4i6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



' ' The same statement may also be made with regard 

 to the majority of Carboniferous shells. There is, how- 

 ever, a notable exception in Coloceras globatum (sp. 

 De Kon.) Hyatt, and ver}' likely some other species 

 of closely coiled nautilian shells. In C. globatum of 

 Vis6, Belgium, I found in seven specimens that the 

 impressed zone appeared while the whorl was still in 

 the cyrtoceran (or open) stage. Fig. iig, Nos. 9-10, 

 give outlines of the adult of this species, and Nos. 11- 

 12, of the young and the zone, showing that the im- 

 pression appeared long before the whorls touched each 

 other and began to assume nautilian characters. Sec- 

 tion, No. \},b, shows the impressed zone occurring in 

 the cyrtoceran stage while the venter or outer side of 

 the whorl was rounded. Such facts admit of but one 

 explanation, namely, that in this species the impressed 

 zone had become hereditary and was in consequence 

 repeated at an early age, previous to the occurrence 

 of close coiling which usually produced it in the ances- 

 tral forms of the same group. 



"There are certain correlative characters which 

 lead me to think that this is only a partial statement and 

 perhaps a more complete and better one would be as 

 follows : that the impressed zone, together with a pe- 

 culiar broadening out of the dorsum and helmet-shaped 

 section of the whorl, and perhaps also certain forms of 

 sutures occurred in the early stages of some Carbonif- 

 erous species before the nautilian stage, and conse- 

 quently they must have been introduced by heredity 

 into the development of this species before the ten- 

 dency to close coiling had completed the first whorl. 



the impressed zone appeared as a genetic character or as a mechanical neces- 

 sity. Either view can be taken, but tlie positive evidence is that they are very 

 rare, and the impressed zone appears in them as a parallel character of dis- 

 tinct diverging series of forms. — A. H. 



