ACROTRETID^. 657 



Acrotretaf subsiduayHBiTE, 1877, U. S. Geog. Surveys W. 100th Mer., vol. 4, pt. 1, pp. 34-36, PI. I,figs. 3a-d. (Described 



essentially as in preceding reference and discussed. The specimens represented by figs. 3a and 3d are redrawn 



in this monograph, PL LX, figs. 1 and In, respectively.) 

 Acrothele subsidua White, 1880, Proc. U. S. Nat. Mus., vol. 3, p. 47. (Merely changes generic reference.) 

 AcrotheU subsidua ("WTiite), Walcott (in part), 1886, Bull. U. S. Geol. Survey No. 30, pp. 108-109, PI. IX, figs. 4a-c 



(not fig. 4). (Original description copied, but the paragraph added on p. 109 includes reference to specimens 



now referred to Acrothele spurri. Figs. 4a-c are drawn from the specimens figured by White, 1877, PI. I, figs. 



3a-d. Fig. 4 is now referred to Acrothele spurri.) 

 Acrothele subsidua (White), Walcott (in part), 1891, Tenth Ann. Kept. U. S. Geol. Survey, pp. 608-609, PI. LXX, 



figs, la-c (not fig. 1). (The text reference mentions this species together with specimens which are now referred 



to Acrothele spurri. Figs, la-c are copied from Walcott, 1886b, PL IX, figs. 4a-c, respectively. Fig. 1 is now 



referred to Acrothele spurri.) 

 Acrothele subsidua (White), Beecher, 1891, Am. Jour. Sci., 3d ser., vol. 41, PL XVII, fig. 12. (No text reference.) 

 Acrothele subsidua ("White), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, pp. 100 and 



103, PL III, figs. 30 and 31. (Mentioned.) 

 Acrothele subsidua (White), Matthew, 1902, Trans. Roy. Soc. Canada for 1902, 2d ser., vol. 8, sec. 4, No. 3, p. 110. 



(Specimens from new locality characterized.) 

 Acrothele subsidua (White), Grabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 200, figs. 234d-e, 



p. 199. (Described. Figs. 234d-e are copied from Walcott, 1886b, PL IX, figs. 4a-b, respectively.) 



General form subcircular to transversely broad oval in outline with the posterior margin 

 broadly rounded to sUghtly transverse in the dorsal valve. Ventral valve slightly convex 

 toward the apex, with the apex between the posterior one-fifth to one-third the distance from 

 the front to the back margin. The apex when perfect is formed of two minute elongate tuber- 

 cles with a narrow elongate depression between them; the size and form of the tubercles and 

 depression between them varies; in some specimens the tubercles are united in front, and in 

 others there is a third minute tubercle just back of the apical depression. A pedicle opening 

 varying from a minute circular to a small elongate-ovate aperture, narrow in front, occurs at 

 the posterior end of the apical depression; it may have its narrow end well between the elongate 

 tubercles or back of them. A triangular false area is weU defined on some shells, and very 

 obscure on others; it is often divided midway by an elevated line which in some examples is 

 a narrow rounded ridge extending from just back of the pedicle opening to the posterior margin; 

 the concentric strige of growth cross the area and its median ridge or line without interruption; 

 casts of the interior show the outline of the false area more or less distinctly. The posterior 

 margin is usually shghtly arched beneath the false area. 



The dorsal valve is depressed convex, with the greatest elevation in front of the posterior 

 margin; the beak is minute, marginal, and divided at the center by an elongate depression 

 that leaves a very minute, shghtly elevated, elongate tubercle on each side. 



The exterior surface is marked by concentric lines of growth that vary in prominence 

 and strength, and some ventral valves have a few low, rounded, rachating costse; the costse 

 differ greatly in length and size, and are entirely absent on many shells. The epidermal layer 

 is ornamented -wdth a minute granulation formed of very minute granules distributed more 

 or less irregularly on irregular raised hues distinct from the concentric strise of growth, or 

 apparently at times superimposed on the latter. The shiny inner layers of the shell are marked 

 by fine, radiating strise and concentric hues and narrow radiating depressions corresponding 

 to the external costse. The shell is corneous and built up of tliin layers or lameUse that give it 

 some tliickness over the visceral portions; the inner lameUaj are slightly oblique to the outer 

 surface. The average size of the shells at the typical locahty at Antelope Springs is from 

 5 to 6 mm. in length by 6 to 7 mm. in width; a few miles distant and a httle liigher in the 

 strata, shells occur 8 to 10 mm. in diameter. Near Montpelier, Idaho, shells from 13 mm. in 

 diameter to young shells 3 mm. in diameter occur in the same bed of shale. 



In the interior of the ventral valve a narrow main vascular sinus arches outward from each 



side a little back of the pedicle opening, and then forward into the anterior half of the valve. 



The pecUcle opening is elongate oval in outhne, and has a slightly elevated rim about it and 



on each side a depression corresponding to the elongate tubercles on the exterior of the shell; 



62667°— VOL 51, pt 1—12 42 



