124 



Report of the State Geologist. 



Bactrites gracilior, sp. now 



Plate IX, Figs. 1-16. 



1891 Bactrite& ) sp. nov. Clarke. Neues Jahrb. fur Min., Bnd. 1, p. 166 ; Amer. 



Geo!, August, p. 95. 

 1894 Bactrites cf. gracilis (Sandberger), Clarke, Amer. Geol. vol. xiv, p. 37. 



In its adult form this species has a somewhat less gradual expansion of 

 the conch, usually a less length and a less elliptical cross section than its asso- 

 ciate B. aciculum. The shell when quite uncompressed is sometimes subcir- 

 cular in section, as is well shown by the larger barite replacements, but speci- 

 mens from the calc-nodules may have their ellipticity of section somewhat exag- 

 gerated. The angle of divergence of the shell walls in the final parts of the adult 

 would indicate a complete length of about 180 mm. for an average individual. 



Body-chamber. The final chamber in two individuals retaining the aper- 

 ture, has a depth of 30 mm. At about the middle of its length it swells slightly 

 and thence gradually contracts forming a decided constriction. At the aperture 

 the shell is expanded and wider than in any other place, so that the direct 

 slope from the aperture is broadly funnel-shaped. The margin of the aperture 

 may be slightly oblique and curve backward a little at the sides, but no 

 specimen exhibits this feature in its entirety. 



Air-chambers. The septal chambers decrease in relative depth from early 

 growth stages, and though this modification is slight, it appears to be uniformly 

 gradual. In the adult condition we count four air-chambers in a length of 

 1 mm. This may be contrasted with the early chambers where in a distance 

 of 5 mm. from the protoconch there are ten septa. Yet this statement of 

 relative decrease in depth of these chambers is true. 



Septa. The septa are quite regularly convex, considerably oblique, slop- 

 ing toward the dorsum. The septum meets the wall of the conch at a very 

 small angle and thus opens a possibility of apparent slight variation in the 

 course of the suture as preserved on the internal cast. The dorsal lobe is 

 well marked and on internal casts almost invariably exaggerated by the 

 exfoliation of the thin edge of the septal cast. The sipho and 

 siphonal funnels are not in any case or at any period of 

 growth, distinctly marginal but are bounded dorsally by a 



Figure 101. Diagram . . . . „ 1 • l 



mstic figure showing the clear uioietv of the septum, lhis is an important tact which 



relation of the tiphonal J x x 



tu>,r>ei to t e uin.r sh.n g^^jfl j je emph asized in any attempt to determine the 

 phylogenic status of this genus. The "dorsal lobe" of Bactrites is virtually 

 a condition due to the. detachment of the edge of the shallow septal cast and 



