(General characters of cephalopods. 
764 THE PALEONTOLOGY OF MINNESOTA. 
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 Roemer). 
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 reguiar succession of siphonated septa 
as in Cyrtoceras, but this mode of growth is abruptly terminated in later development 
by a lapse toa 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 Lindstrém), 
*It is elsewhere observed that in the Devonian and typical representatives of this genus this swelling of the conch is 
usually absent, but it is more or less distinctly developed in the majority of the early Silurian species, 
