598 
dorsal furrow similar to that which appears on the dorsum of the 
nepionic stage. The section of this whorl is nephritic. The ap- 
pearance of the dorsal furrow is very often in the young and in later 
stages of growth correlated with the appearance of a nephritic out- 
line in the whorl. This happens So often that I at first supposed it 
was a general law of association the two appearing together. It is 
true, that in a number of forms, the nephritic form appears in asso- 
ciation with the dorsal furrow, but in quite a number of others the 
outline is not nephritic, and yet a dorsal furrow arises as will be 
noted farther on. 
The large size and gradual curvature of the cone in this genus 
makes it unlikely that the existence of the dorsal furrow is due to 
contact or to any mechanical effect of coiling. ‘The dorsal furrow 
in these is either due to inheritance from other species, or is acquired 
‘in their later or ephebic stage. 
The genus may be degenerate and may have arisen from coiled 
forms and the dorsal furrow and nephritic outline may have been 
derived from this source. Against this is the fact that the shells are 
of large size and the septa are.closely approximate. Both of these 
characters are common in primitive Paleozoic shells and uncommon 
in degenerate phylogerontic series. The study of the fossils them- 
selves does not seem to support this view of their affinities since it 
is difficult to point to any preéxisting coiled form from which they 
could have been derived. If it is assumed that they are primitive 
arcuate forms descended from other arcuate forms or straighter 
cones, it is easy to trace them back into the Silurian and point out 
their probable ancestors, in closely allied species which do not have 
a dorsal furrow. 
The problem here assumes a very interesting character due to the 
fact that the Silurian forms of Cranoceras, C. ¢urnus, and others 
have trigonal whorls and sutures which are in every way identical 
with the young of several nautilian shells of the same period and 
are evidently their ancestral radicals. These arcuate species, how- 
ever, do not have dorsal furrows, and it seems, therefore, highly 
probable that here is a case of acquired characteristic coming in very 
late in the ontogeny of the ephebic stage, accompanied by a nephritic 
outline. 
Contact furrows arise from close coiling in fossils like Wedyceras 
vetustum having similar subtrigonal whorls, but no examples are 
