Monographs of the United States Geological Survey Volume 42

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated.1903 Excerpt: ... Family BACTRITID.E. Genus Bactrites Sandberger. Shell straight or slightly curved, gently tapering, cross section circular or ellipticaI. Surface smooth except for the cross striae of growth. Siphuncle marginal, with long backward-pointing extensions of the septum, as in Spirilla. Septa simple, running nearly straight around the shell, but with a funnel-shaped siphonal lobe. Bactrites was formerly regarded as a nautiloid, but its siphonal lobe and the calcareous protoconch that has been found on a few specimens cause it to be classed usually under the ammonoids. The genus has been usually considered as typical of the Devonian, and the species described below is the only authentic species known to occur in Carboniferous strata. Bactrites Carronarius Smith, sp. nov. PI. VI, figs. 9-11. Shell straight, gently tapering, slender, angle of the increase 5 30'. Cross section of whorl circular. Chambers convex backward, deep, septa close together. The septum runs nearly straight around the shell, but has a slender tongue-shaped siphonal lobe, and short backward pointing siphonal collar. Siphuncle slender, with long backward extension of the septum, as in Spirula; when this extension reaches the extremity of the siphonal lobe it swells out into a knob-like expansion, supposed to represent a periodic repetition of the siphonal caecum. From this contact with the septum a short siphonal collar extends backward into the next chamber. This septum is much more specialized than that of any other species of Bactrites yet described, but this is not remarkable if we consider the fact that the genus has not before been found above Devonian bods. Surface smooth except for fine sinuous imbricating cross striae of growth. Bactrites was probably the ancestor of the entire ammoiioid...