Geol.— Vol. I.] SMITH— PLACENTICER AS. 211 



gastropods, and only in later growth was the spiral formed. 

 It has, however, been shown by Dr. Amos Brown (1892) 

 that in Baculites the limits of the embryo chamber lie 

 between the first and second septa. This has also been 

 observed by the writer on the young of Baculites chico'ensis, 

 and on Lytoceras almuedense. 



The protoconch of Placenticeras facificum (PL XXIV, 

 figs. 1-3) has diameter 0.54 mm., and width 0.75 mm.; it 

 is smooth, oval, and covered by the primitive nacreous 

 shell, which extends to the end of the first coil. This pro- 

 toconch is very similar in all the later ammonites, and is 

 probably an adaptive form, due to life in the ^^'g, and 

 does not represent any ancient ancestral genus, for none 

 of the early cephalopods were shaped like this. It is, 

 then, the typical embryo of the ammonoids, and yet can 

 hardly be said to be correlative with any group of ceph- 

 alopods. 



Ananef ionic. — With the formation of the first septum 

 the young ammonite has taken its place among the cham- 

 bered cephalopods, and has become, for the time being, a 

 nautiloid, although it is not possible, from the exceedingly 

 simple nature of the shell, to correlate it with any especial 

 genus. Nor, indeed, is it strictly homologous with any 

 ancient nautilian form, for the larval ammonite even in its 

 first stages possesses several elements unknown in that 

 group. The first septum, which separates the larval body- 

 chamber from the embryonic shell, is nautilian in character, 

 but the siphuncle begins inside the protoconch with a 

 siphonal knob, or cascum, and the protoconch itself is cal- 

 careous. These are two characters that the nautiloids, even 

 to this day, have never yet acquired. It would, then, be 

 impossible to correlate the ananepionic stage with any 

 ancestral genus, since we have in this stage ammonoid char- 

 acters pushed back by unequal acceleration, until they 

 occur contemporaneously with more remote ancestral char- 

 acters. This stage and these characters can not correctly 

 be called adaptive, for they are undoubtedly hereditary, 

 although not inherited at equal rates. 



