597 
ated the single or double dorsal furrows of Zrocholites canadensts, 
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 Eatont 
in one specimen, Pl. vi, Figs. 28-35, and in another it is present 
for a longer time after the volution becomes free, although evidently 
much reduced, Figs. 7 and 8, Pl. vii. In Schroederoceras casinense, 
Pls. 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. Hurystomites Kelloggt, 
Pl. 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 Tarphyceratidz and Trocholitidz 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 
