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apex, and at least as long ventro-dorsally as the same. In other 

 words, the aperture of the protoconch in Nautiloidea was narrow 

 and elongated vertically, while that of the Ammonoidea in all hav- 

 ing cylindrical, straight or loosely coiled young shells, was an open 

 tube, as happens in Clarke's Orthoceran form, in Bactrites and in a 

 number of Goniatitinae as shown in the figures. 



In most groups of Goniatitinae and the other suborders of Am- 

 monoidea which, as a rule, have invariably closely-coiled first whorls, 

 the effect of contact is to produce immediately a deep, contact fur- 

 row and an almost entire obliteration of the umbilical perforation 

 between the neck of the protoconch and the nepionic volution. 

 Two funnel-shaped openings are left on either side, as shown in fig- 

 ures on PL ii, and these represent the more complete perforation 

 present -in all Nautiloidea and in the earliest forms of Goniatitinae 

 among Ammonoidea. The probable position of the aperture of 

 the protoconch has been indicated in Embryology of Fossil Cephalo- 

 pods, p. no, and in PL iv, Fig. i, and this information, gathered 

 from sections, agrees well with the figure given by Dr. Brown of the 

 supposed aperture of Baculites which is reproduced in outline, Fig. 

 17, PL ii.* 



The growth of this form out of the protoconch, as in Bactrites, 

 must have been quite different from that of the true Nautiloidea. 

 Nevertheless it is obvious that as the animal grew outside of the 

 limits of the protoconchial aperture, it began to build the shell of 

 the apex of the conch and the first living chamber. This was the 

 ananepionic substage and it in part more or less resembled in some 

 of its essential characteristics and for a short time, the aseptate, 

 apical living chamber of the Nautiloid, but this resemblance must 

 have been transient and much accelerated. 



After or during the building of this external skeletal tube it 

 became practicable for the animal to lift itself, or, more properly 

 speaking, to progress by growth out of the protoconch, and the 

 next step can be seen in Branco's Fig. 10, PL iii, and the details in 

 my Fig. 7, PL iii, both of which, and others also given, show that 

 the bottom of the caecum occupied the aperture of the protoconch 

 and is formed, as in Nautiloids, of the closed funnel of the first 

 septum. It is therefore inherited earlier, according to the law of 

 tachygenesis, since the first septum and the caecum occupy the same 

 position with relation to the protoconch as the scar or cicatrix in 



* Proc. Acad. Sci. Phil, 1892, PI. ix, Figs. 5 and 10, 11. 



