BRACHIOPODA. 79 



Remarks — This is a rare form in the Mineola and the individuals are always small, 

 the maximum size collected measuring less than an inch along the hingeline. In the 

 Callaway it is common and a tendency toward variation in several directions has ap- 

 peared. In the Snyder Creek it is very abundant and S. boonensis Swallow, 5. callaway- 

 ensis Swallow, S. cymbiformis Swallow, 5. navalis Swallow, 5. infiexa Swallow and 5. 

 equicoslata Swallow developed from it in a short time. None of the new species lived 

 beyond Snyder Creek time. More than 1,000 specimens of these stropheodonts are in 

 the collections of the University of Missouri and the variations present are remarkable 

 as they all took place while 25 feet of shales and limestones were being deposited. 



Occurrence — Mineola of Warren, Montgomery, Ralls and Callaway Counties; Calla- 

 way of Warren, Montgomery, Callaway, Boone and Moniteau Counties; Snyder Creek 

 of Montgomery and Callaway Counties. 



Stropheodonla equicostaia Swallow 



Plate 11, figure 6; plate 12, figures 9-17. 



1860. Strophodonta aeqiiicostala Swallow, Trans. St. Louis Acad. Sci., I, p. 639. 

 1860. Strophodonta quadrata Swallow, Ibid., p. 639. 



1897. Stropheodonla callawayensis, quadrata and aequicostata Schuchert, 

 Bull. U. S. Geol. Surv., 87, pp. 420 and 425. 



Swallow's description — "Shell suborbicular, auriculate, with a sinus on each lateral 

 margin anterior to the ears. Ventral valve subsemihemispherical, regularly convex, 

 but most arched between the beak and the center; ears defined by a depression on this 

 valve and the sinuses on the lateral margins; they are arched and but slightly change 

 the regular contour of the valve; area straight, narrow, vertically striated, and common 

 to both valves; umbo depressed; beak small, depressed. Dorsal valve very concave, 

 following closely the curvature of the opposite valve, much flattened towards the 

 cardinal border; lateral edges depressed and arched to form the concave surface of the 

 ears; cardinal border slightly arched and finely crenulated. Surface marked by numerous 

 small, rounded, granulated, radiating plications, and by very fine concentric striae, 

 and imbricating plications of growth. 



Length, 1.18; breadth, 1.45; depth, .53; cardinal line, 1.20." 



Supplementary description — The specimens which come nearest to other parts of 

 Swallow's description do not have projecting auriculations and as his measurements 

 show the hingeline about 1/6 shorter than the maximum width of the shell, he probably 

 meant only that the auriculations are distinct. The sinus anterior to the ear is not 

 present on the most typical specimens of the group but occurs on a variety which grades 

 towards 5. cymbiformis Swallow. The most typical examples have the hingeline about 

 the same width as the shell but the variations seem to be towards shorter hingelines in 

 every case. Three typical examples give the following measurements: Length, 24, 

 24, 26 mm. ; length of hingeline 26, 28, 26 mm. ; convexity 12, 12, 143^ mm. The greatest 

 width of specimen 3 is 2 mm. more than the width of the hingeline. One specimen of the 

 cymbiformis type has a hingeline length of 17 mm. and a greatest width of 21 mm. 

 A specimen with a hinge length of 13 mm., and a maximum width of 19 mm. is inter- 

 mediate between S. cymbiformis Swallow and 5. equicoslata Swallow. The fine plica- 

 tions are rounded to subangular and vary from 14 to 20 to the centimeter. None of the 

 specimens has fine concentric striae though a few lines of growth are present. The 

 plications appear granulated on account of numerous very small pores. 



The varieties of this species grade toward S. cymbiformis Swallow but the typical 

 forms are quite distinct and none of them has the umbo projecting prominently back 

 of the hingeline. In another direction it grades toward 5. demissa (Conrad) but differs 

 from it in the much more strongly arched valves and shorter hingeline. 



Remarks — Schuchert uses the name 5. callawayensis Swallow for this species, but 

 callawayensis can not be positively identified among the Snyder Creek specimens from 

 Swallow's description and equicoslata is easily identified. No specimens among our 1,000 



