Bekdb.] Carboniferous Invertebrates. 65 



much more rapidly than does the hinge after the shell reaches 

 middle age. 



Professor Clarke writes me concerning these two species that 

 "There can be no question of the close similarity of these two 

 species, and D. robusta was founded upon a single specimen 

 remarkable for the convexity of its brachial valve and its sub- 

 circular outline, but I recollect distinctly that we had a com- 

 paratively meager representation of this species from the Coal 

 Measures. T have little doubt that one represents the continu- 

 ation of the other specific type, perhaps with some slight varia- 

 tion, and that the genus attained its culminant variability of 

 expression in the Coal Measures." 



The relative convexity of the brachial valve varies to a con- 

 siderable degree, though the most of them are very convex. Con- 

 sidering these points as a whole, the only grounds left, so far as 

 I can see, for J), robusta to rest upon, are its relatively little 

 longer hinge and greater convexity of the pedicle valve, both of 

 which are variable characters, and the two species approach each 

 other very closely in this respect. However, it may be suffi- 

 cient to distinguish them, though J think that they will prove 

 to be the same. 



MEEKELLA. 



Whit* and St. John, Trans. Chic, Acad. Sci. I, p. 120, ff. 4-6, (1863). 

 Hall and Clarke. Pal. N. V. VIII, pt. I, p. 264 (1892) ; etc. 



Meekella striatocostata. Plate XII, tigs. 9-9c. 



Plicatula striatocostata Cox, Geo!. Surv. Kv., 1 1 r , pt. i, p. 568, pi. viii, 



f. 7, (1857). 

 Orthishia shumardianus Swallow. Trans. St. L. Acad. Sci., i, p. 183, 



(1858 i. 

 Orthisina missouriensis Swallow, ibid., p. 219: etc. 

 Orthis striatocostata Geinitz, Carb. u. Dvas in Neb., p. 18, pi. in, ff. 22- 



21, (18(56). 

 Meekella striatocostata White and St. John, Trans. Ohio. Acad. Sci., pp. 



129, 122, ff. 1 6 (1868i: Meek, Fin. Rep. U. S. Geol. Surv. Neb., p. 175, 



pi. v, f. 12, (1872 i : etc. 

 Streptorhynchvs [Meekella) striatocostata Hall, 2d Ann. Rep. N.V.St. 



Geol., pi. xl, ff. 18-23, (1883i. 



Meek's description < in part ) : " Shell trigonal-subglobose, be- 

 coming very convex with age, generally a little longer than 

 wide ; hinge lino very much shorter than the breadth of the 

 valves. Dorsal valve convex, the greatest prominence near the 



