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ated the single or double dorsal furrows of Trocholites canadensis, 

 or that heredity influenced the appearance of both, it would be 

 necessary to find more forms of the same genetic series and study 

 their history. 



In some species of the genus Schroederoceras, the dorsal fur- 

 row appears as in Trocholites. The umbilical perforation is larger 

 but still small in all of these, so that it can hardly be assumed that 

 the bend is too gradual to have caused the dorsal furrow to arise in 

 the paranepionic. 



The gerontic stage of the species of this family, in fossils well 

 enough preserved to be observed, has an impressed zone which is 

 very short-lived in some species when the last whorl is free. The 

 entire obliteration of this zone takes place in Schroederoceras Eatoni 

 in one specimen, PI. vi, Figs. 28-35, an< ^ i* 1 another it is present 

 for a longer time after the volution becomes free, although evidently 

 much reduced, Figs. 7 and 8, PI. vii. In Schroederoceras casinense, 

 Pis. vi and vii, similar obliteration can be observed. 



The zone, however, persists long enough in these forms and 

 others to demonstrate the important fact that it has a deep hold 

 upon the organism. If this were not the case it could not exist in 

 substages of senile degeneration. Its persistency is somewhat less 

 in the species cited than many others, ex. Eurystomites Kelloggi, 

 PI. v, but it is sufficient to show that its continued existence in the 

 ontogeny is not wholly limited by the continuance of close coiling 

 and contact. That it is more or less dependent upon coiling with 

 involution is obvious because it entirely disappears in some species 

 in the older substages of the gerontic stage when these are free. 



The Tarphyceratidse and Trocholitidas having so closely involute 

 shells in the young are confined, with the exception of Trocholites, 

 to the earliest or Calciferous faunas. 



The next forms that one meets, having the impressed zone, occur 

 in the Devonian. There are so far as known no shells having an 

 impressed zone in the form of a dorsal furrow between the Hudson 

 River group and the Devonian group, although there are many having 

 the contact furrow. 



The Devonian genus Cranoceras, referred to several times above, 

 consists of two species with very large shells, and, so far as can be 

 seen, purely arcuate forms, is the only case of a cyrtoceran form 

 with a dorsal furrow that I have been able to find. The zone in 

 this shape appears on the free inner or dorsal side and is obviously a 



