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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- 

 tilinidae, have in varieties of two species, as shown by Barrande, a 

 straight apex, like the adult shell of such forms as Bactrites f 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. Mimoceras 

 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- 

 tilinidse, 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, 

 PI. ii. 

 f A straight form of Goniatitinse (see Figs. 30, 31, PI. 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 

 passiug on to its surface from the conch, 

 which are made somewhat more promi- 

 nent in the figures than iu nature, in 

 order to demonstrate this connection. 

 The ananepionic substage is smooth and 

 distinctly marked off from the succeed- 

 ing, probably metanepionic substage, 

 which shows both longitudinal ridges and 

 transverse bands of growth. The metane- 

 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 iu the character of the ridges and bands of growth. 

 The absence of a hyponomic sinus in the young, of stiaight 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. 



Fig. 10. Fig. 11. 



Figs. 10-12, Spyroceras crotalum. 



