764 



THE PALEONTOLOGY OF MINNESOTA. 



[General characters of eephalopods. 



regularly conical shells implies equal progress in the deposition of the shell-matter 

 on all sides, but when the shell is coiled this effect is due to an obstruction of the 

 shell growth on one side. In certain of the coiled genera we know that the enrolled 

 portion of the shell represents only the immature stages of existence, while during 

 later-growth stages the shell becomes straight. 



This fact, illustrated by the genus Lituites, does not necessarily imply that such 

 forms have been derived from primitive coiled types, but may with excellent reason 



Fig. 3. Lituites lituus. (After Rcemer). 



be interpreted as follows: The straight conch of the full-grown animal may be 

 regarded as a senile character expressing a return to, or towards a primitive growth- 

 condition not otherwise represented in the individual, but indicating the source 

 whence these generic traits have been derived. 



In the peculiar genus Ascoceras the early growth of the shell is in the form of a 

 long, very slender, gently arcuate cone with a regular succession of siphonated septa 

 as in Cyrtoceras, but this mode of growth is abruptly terminated in later development 

 by a lapse to a much more elementary condition of development evinced by the 

 suddenly swollen .conch and the incomplete and primitive septa. Several of the 

 genera here considered are characterized by a swelling or expansion of the shell 

 during later growth, and a sudden contraction at the close of the swelling near the 

 aperture. This is observable in Oncoceras, Clinoceras, Poterioceras and the shells 

 referred to Cyrtoceras* The presence of this character in these early types might 

 of itself be interpreted as indicative of primitive structure, as it has recently been 

 shown that in certain Devonian species of the orthoceran genus Bactrites this expan- 

 sion of the shell characterizes the growth stage directly succeeding the formation 

 of the protoconch. 



Fig. 4. Ascoceras manubrium. (After Li ndstn'im). 



It Is elsewhere observed thai In the Devonian and typical representatives iif tills genus tills swelling of the couch Is 

 uiually absent, but It li more or less distinctly developed In the majority of the early Sllurluu species. 



