BACTRITES, b5k-tri'tft (Gk. Aceterpop, bak iron, staff, stick, in allusion to its straight. form similar to that of Baculites). A genus of fossil cephalopods of the order Ammonoidea, and the only member of the family Bactritidx. The shells are straight, very slender, and gently tapering, with a round or elliptical section. The siphuncle is situated near the ventral wall and is very deli cate. The suture is a simple curve with a small ventral angular lobe over the siphuncle, and, in some species of elliptical section, an incipient lateral lobe. The protoconch or embryonic shell is egg-shaped and erect, and has no sear such as is found on the initial chamber of the Nautilus. Bactrites is of more than ordinary interest, because it seems to furnish one of those rare examples of a connecting link between two well differentiated orders of animals; namely, the Nautiloidea and the Ammonoidea. The general form of the shell is that of Orthoceras, and the straight cone of Bactrites is a primitive char acter, as it is in Orthoceras, and not to be compared with the degraded uncoiled shells (Baculites) that are evolved in the senile (phy logerontic) stages of several Jurassic and Cre taceous races of Ammonoidea. The shape and
position of the protoeonch are very similar to that of Mimoceras (Goniatites) compressum, which shell is uncoiled in its early embryonic stages, but becomes closely wound in its adoles cent stages. Moreover, one of the Nautiloidea, the genus Protobactrites, seems to afford an in dication of the stock from which Bactrites itself was derived.
The genus Bactrites contains about fifteen spe cies that range through all the formations from the Ordovician to the Trias, inclusive, with the exception of the Silurian. in which latter no members of the group have yet been found. See CEPHALOPODA.