t2 The American Geologist. July, 1894 
be observed that the last septa of Oncoceras crossing the ex- 
panded portion of the tube are frequently crowded together 
in a closer juxtaposition than elsewhere in the shell. Such a 
rapid recurrence of the septa is construed by Hyatt as an 
evidence of degeneracy or senility. 
This expansion is less developed in B. aciculum, but its 
presence is evident. In Branco's figure, however, there is no 
such feature, the tube in the vicinity of the protoconch being 
narrower than at any other section. 
Ornamentation. The fine, oblique thread-like lines which 
cover the surface are readily distinguished to the very base of 
the protoconch (figure 4). Toward the later parts of the shell 
they become more distant, gradually less distinct, and near 
maturity are lost or extremely obscure, becoming merged into 
the low, oblique undulations of the surface over the body 
chamber of the mature individual. In B. aciculum there are 
periodic internal annular thickenings of the shell substance, 
which are even manifested on the minute example shown in 
figure 9 (an internal cast in pyrite), but similar developments 
have not been observed in B. gracilis. 
Whatever may be the apparent discrepancies in Branco's 
determination of the Hurt rites protoconch and that here given, 
it may at least be said for the material here studied that it 
corresponds, even to its specific characters, with that upon 
which the genus was established. If the evidence presented 
brings us to the conclusion entertained by many of the older 
palaeontologists, that Bactrites is closely related to Orthoceras, 
this conclusion is attained, by means of data not before elab- 
orated, namely, the existence in both of like protoeonchs. 
This fact, fortified bj" the decisive evidence that in Bactrites 
the sipho is strictly intra-marginal, the formation of a dorsal 
lobe wholly casual, leads to the conviction that Bactrites is 
little else than an orthoceran nautiloid with a lateral sipho. 
Both Branco* and Hyattf have suggested the probability of 
Belemnites having been derived from orthoceran stock. Hyatt 
demonstrates that the guard or rostrum in this genus is a hy- 
pertrophic secondary deposition about the earlier parts of the 
*Palaeontographica, ut. <-it. 
+ Pt-oc AmiT. Assoc. Adv. Sci., vol. 32, p. :i.'i7. 1884; and Science, vol. 
'.\. i>. 1-2). 1884. 
