361 
forms of Cephalopoda—-had a common origin, probably in some 
chamberless and septaless form similar to the protoconch. 
Clarke has recently shown that a straight, Orthoceras-like shell 
may have a complete egg-shaped protoconch like that of Bactrites.* 
His form certainly has the characters of an Orthoceras, but the 
protoconch is large and like that of the Ammonoidea. The shell 
may be transitional from Orthoceras to Bactrites, but is probably 
not a typical form of Orthoceras. 
The young of the simplest and earliest of Ammonoidea, the Nau- 
tilinide, have in varieties of two species, as shown by Barrande, a 
straight apex, like the adult shell of such forms as Bactrites + and 
that described by Clarke. I have already claimed that this fact was 
sufficient to prove the high probability of a common origin from a 
straight shell like Orthoceras for both of the orders. A/imoceras 
compressum, sp. Beyrich (Figs. 1-6, 20, Pl. ii), is a shell which 
differs from all other Ammonoidea in an essential and highly impor- 
tant character. The septa have no inner lobe. The V-shaped 
annular lobe, which occurs in all the Ammonoidea except the Nau- 
tilinidee, is also absent in this species. What is more to the point, 
some species have the sutures of a true nautiloid, since they have 
* “The Protoconch of Orthoceras,’’ Am. Geol., xii, Aug., 1893. See also Figs. 28, 29, 
Pl. ii. 
+A straight form of Goniatitine (see Figs. 30, 31, Pl. ii). 
* Prof. Hall, in his Paleontology of New 
York, described a young specimen of 
Spyroceras (Orthoceras) crotalum, sp. Hall, 
which he subsequently loaned me for fur- 
ther study. Upon developing the speci- 
men, I found the beautifully preserved 
apex shown in Figs, 10-12. This shows 
the shriveled protoconch with striations 
passing on to its surface from the conch, 
which are made somewhat more promi- 
nent in the figures than in nature, in 
order to demonstrate this connection. 
The ananepionic substage is smooth and 
distinctly marked off from the succeed- 
ing, probably metanepionic substage, Fig. 10. Fic. 11. 
which shows both longitudinal ridges and 
transverse bands of growth. The metane- Figs. 10-12, SPYROCERAS CROTALUM. 
pionic substage is marked off below by a more prominent band of growth, probably 
indicating the aperture of this substage. The paranepionic substage below this 
changes in the form of the cone and in the character of the ridges and bands of growth. 
The absence of a hyponomic sinus in the young, of straight as well as of nautilian shells, 
shows that they were not active swimmers in these earlier nepionic substages, and that 
the hyponome was acquired or at any rate large and functionally active only at a com- 
paratively late age of the ontogeny. 
PROC. AMER. PHILOS. SOC. XXXII. 148. 2 T. PRINTED MAY 25, 1894. 
