I)Kedk.] Carboniferous Invertebrates. 57 



CRANIA. 



Retzius, Schrift. Ges. Naturf. Freuende. Berlin, II, p. 72, (1781). 

 Hall and Clarke, Pal. N. Y., pt. I, p. 145, (1882) ; etc. 



Crania modesta. Plate VIII, fig. 4. 



Crania modesta White and St. John, Trans. Chic. Acad. Sci., i, p. 118, 

 (1868): White, 13th Rep. Ind. St. Geol., p. 121, pi. xxxv, f. 9, pi. xxxvi, 

 f. 5, (1884). 



Crania carbonari a Whitfield, Ann. N. Y. Acad. Sci., n, p. 229, (1882): 

 Geol. Ohio, vn, p. 484, pi. xi, ff. 11, 12, (1895): etc. 



Whitfield's description : " Shell small, none of the specimens 

 observed exceeding three-eighths of an inch in diameter ; subor- 

 bicular in outline, or varied in form by the outline of the objects 

 to which they are attached. Free valve depressed convex, 

 marked by a few concentric lines of growth ; attached valve 

 thin, but with a slightly thickened margin. Posterior muscular 

 impressions large and submarginal, the others being nearly 

 central and forming a small elevation just posterior to the 

 middle of the valve. '' Measurements: Diameter, 9 mm.; 

 height, 3 mm. 



Range and distribution : Upper Coal Measures ; Kansas City, 

 Eudora, Grand Summit. 



The specimens are attached to Product"* semireticulatus , and 

 the convex valves reproduce the markings of the host. One 

 near the front of the shell shows the large radiating costa' quite 

 plainly, while on the reticulated area shows both the radiating 

 and concentric marks, while still another situated partly on the 

 reticulated area shows the reticulated markings on one side and 

 only the radiating on the other. Two specimens on worn shells 

 show none of the markings of the host. The attached valves 

 are thin, but show the muscular markings quite distinctly. 



DERBYA. 



Waaler). Pal. Indica, ,-er. XIII, I, pp. 576, 591, (1884). 

 Hall and Clarke, Pal. N. I., VIII, pt. I, p. 261, i 1892 I : 11th Add. Rep. X. Y. St, Geol.. 

 p. 286, (1895 i, for 1M94. 



The genus Derbya has been divided by Waagen into groups, 

 but 1 s^e no sufficient reason for such divisions for our American 

 shells. 



A consideable stre-- ha- been laid upon the nature of the 

 hinge area in the different American species of the genus. I 



