FOSSILS OF THE CAMBEIAN. 



17 



In the larger valves the conical form and siphonal tube strongly relate them. 

 (See plate i, figs. 1 6, I/, and fig. 8.) 



The interior of the smaller valve shows a wonderful similarity to that 

 of Obolella f ficabrince Callaway (Dav. Mon. Brit. Foss. Brach., vol. v, pt. ii, 

 p. 211, pi. xvi, fig. 2 7 d\ (see plate 1, fig. lc,) and I can hardly understand 

 that they belong to separate genera. If the conical valve of 0. f Scabrina 

 should be found to show indications of a perforate apex, the relations of the 

 species to Acrotreta gemma would be very close. 



Acrotreta gemma Billings. 

 Plate i, figs, la, If, Id-/; plate ix, figs. 9, 9 a. 



Acrotreta gemma Billings, 1865. Palaeozoic Fossils, vol. i, p. 216, figs. 201 a,/. 

 Acrotreta subconica and A. attenuata Meek, 1873. Sixth Ann. Eep. TJ. S. Geol. Surv. 



Terr., p. 463. 

 Acrotreta pyxidicula White, 1874. Expl. and Surv. West 100th Merid., Eep. Invert, 



Foss., p. 9. Ibid., vol. iv, p. 53, pi. iii, figs. 3 o-d, 1875. 



The specific identity of the forms from Nevada with those from New- 

 foundland scarcely permits of question if based on the description and illus- 

 trations given by Mr. Billings. Some specimens have the conical valve 

 more or less elevated, but the difference is not of specific value. 



Dimensions of an average specimen: height of conical valve, l.TS 111 ; 

 transverse diameter, 2 mm ; longitudinal diameter, 1.75 mm . 



The species from the passage beds between the Cambrian and Silurian 

 faunas on the Gallatin River, Montana, doubtfully identified by Mr. Meek 

 as A. subconica Kutorga, and in case of eventually proving to be a distinct 

 species to be called A. attenuata, proves on comparison to be the same as the 

 Eureka species, and it is also associated with Iphidea sculptilis, Agnostus bidens, 

 and Ptychoparia Oweni, at each locality. The mesial furrow on the false 

 area is a variable character, as also within somewhat broad limits the depres- 

 sion or elevation of the conical valve. With the specimens before me for 

 comparison, they are all referred to the one species and identified with A. 

 gemma. Acrotreta pyxidicula White, from Nevada, occurs at about the same 

 geologic horizon. The smaller valve is identical with that of A. gemma; 

 2 o D w 



