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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 preexisting 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. turnus, 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 Nedyceras 

 veiustum having similar subtrigonal whorls, but no examples are 



