METAZOA MOLLUSCA. 257 



In the history of the development of Baculites we have 

 positive proof of the reduced character of the genus. In- 

 stead of being a relative of the primitive straight Ortho- 

 ceras or of Bactrites, it is an extremely specialized form 

 whose ancestors in their evolutionary history passed 

 through the various stages of shell development. Of 

 these stages the straight and loosely coiled stages are 

 omitted in Baculites, so that the animal begins life with 

 a tightly coiled shell, which, however, ceases to coil in 

 the neanic stage, forming thereafter a straight cone. 



DlBRANCHIATA. BELEMNITIDAE. 



The Belemnitidae begin in the Triassic period and are 

 therefore more recent than the Nautiloids or the Ammo- 

 noids. We should expect to find them more specialized 

 though still retaining certain characters of their ancestors. 



We have seen in Nautilus the dorsal fold of the mantle 

 which secretes the black pigment layer. In Aulacoceras 

 (PI. 626, figs. 1-3, A. reticulatum Hauer), the dorsal fold 

 was nearly closed and secreted a shell with three princi- 

 pal parts: the chambered shell or phragmacone (fig i, 

 ph] ; the guard (fig. !,,"; fig. 3), and a prolongation of 

 the phragmacone, the pro-ostracum (see upper part of fig. 

 i), often called the pen, which is seldom preserved. The 

 phragmacone has chambers, septa, and siphuncle (fig. 2, 

 section). It corresponds with the shell of the Tetra- 

 branchiata, but differs from the latter by being internal. 



The more specialized form, Belemnites (No. 627), has 

 a conical, chambered shell, at the anterior end of which 

 on the dorsal side there was the shovel-like projection or 

 pro-ostracum. The septa of the shell were plain (No. 

 627), concave, and were pierced by a siphuncle which 

 does not show in the specimen. This shell is external 

 in the young Belemnite. In time the dorsal flap of the 

 mantle grows out and around the shell, its two edges not 



