CEPHALOPODA. 187 
Orthoceras bilineatum.] 
The surface is ornamented by coarse and fine vertical, elevated lines, reticulated 
by extremely fine horizontal lines. Toward the apex, over the smooth portion of 
the shell, the vertical lines occur in two simple series; where the shell has a diame- 
ter of 7 mm. there are twelve lines of the first order, between each two lying one of 
a secondary series. As growth advances these lines rapidly multiply by intercala- 
tion, and the alternation in the size of the strize becomes decidedly less pronounced. 
Over the annulated and later portions of the shell the ornamentation becomes 
proportionally very much finer but the regularly alternating size of the lines is 
maintained throughout. The horizontal strize are exceedingly fine and often not 
retained. Where crossing the other series they are usually elevated into slight 
nodes or projections. 
Sipho small and nearly central. The septa are rather shallow and the sutures 
regularly transverse and without undulations. They bear no definite relation to the 
annulations. Over the early, smooth portion of the shell they appear to be relatively 
distant on account of the narrowness of the shell, there being seven air-chambers in 
a length of 17 mm., in another specimen five ina length of 12 mm. They do not 
greatly vary in depth with the increase in the diameter of the shell. The sutures 
being usually transverse, cross the more or less oblique annulations and constrictions, 
variously transecting, or at times lying wholly within a given furrow. 
The original description of this fossil was based upon specimens showing only 
the adult characters of the species. The existence of specimens in the material in 
hand, showing in a single example the gradual change from a smooth to an annu- 
lated shell, brings out an interesting fact in regard to the morphic variation through 
which other annulated species are known to pass. It has, for example, been shown by 
Hall* that the embryonic tip of the shell of Orthoceras crotalum, an annulated 
Devonian species, is smooth, and also that the vertical striz are well developed 
much before the appearance of the annulations. In that species, however, the 
smooth portion of the shell is very short and greatly abbreviated in comparison with 
that of O. bilineatum. The passage of the shell of O. crotalum through the smooth 
stage is highly accelerated, while its longer duration in O. bilineatum more forcibly 
suggests the phyletic as well as individual relation of the non-annulated to the 
annulated forms of this genus. 
It is, however, to be observed that the degree to which the apical smooth shell 
of O. bilineatum is retained is in a certain sense an individual peculiarity. Some 
specimens develop the annulations much earlier than others, and those which retain 
the smooth shell to a considerably later period preserve for a longer period an 
infantile character. 
* Paleontology of New York, vol, v, pt. ii, pl. Cx1u, fig. 13. 
