DESCRIPTIONS OF THE SPECIES. 



Family BACTRITID.E. 

 Genus Bactrites Sahdberger. 



Shell straig'ht' or slightly curved, gently tapering, cross section circular 

 or elliptical. Surface smooth except for the cross stri;« of growth. 

 Siplumele mai'ginal, with long backward-pointing extensions of the septum, 

 as in Spiruki. Septa simple, running nearly straight around the shell, hut 

 with a funnel-shaped si phonal 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 carbonarius 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 sliort backward pointing 

 siphonal collar. Siphuncle slender, with long backward extension of the 

 septum, as in Spinila; 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 c?ecum. From this contact 

 with the septum a short siphonal collar extends backward into the next 

 chamber. This septum is much niore specialized than that of any other 

 species of Bactrites yet described, Ijut this is not remarkable if we consider 

 the fact that the genus has not before been found above Devonian l)eds. 

 Surface smooth except for fine sinuous imbricating cross stria; of growth. 



Bactrites was probably the ancestor of the entire ammonoid stock, and 

 thus in it we have a survival of a primitive type occurring along with the 



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