Beedk.] Carboniferous Invertebrate*. 137 



tremity ; anterior margin narrowly rounded ; posterior extremity 

 more compressed, and more broadly rounded, sometimes a little 

 oblique above ; cardinal margin somewhat straightened along 

 the middle, hut rounding imperceptibly into the anterior and 

 posterior extremities ; beaks much depressed ; or scarcely dis- 

 tinct from the cardinal margin, moderately convex, and placed 

 very near the anterior margin, but not terminal ; umbonal slopes 

 forming a very obscure narrow ridge, which extends, with a 

 slight curve from each umbo, to the posterior basal margin. 

 Surface marked with moderately distinct lines of growth, which 

 on the posterior dorsal region above the umbonal ridge are 

 crossed by Very minute or microscopic radiating and rather dis- 

 tinctly divericating striae. Length of the largest specimen seen, 

 1.03 inches; height of the same, 1.43 inches; convexity, about 

 0.25 inch." 



Uange and distribution: Upper Coal Measures; Topeka. 



MYALINA. 



de Koninck, Desc. Anim. Foss. Carb. Belg., p. 125, (1844). 



Myalina swallovi. Plate XVI, fig. 7. 



Myalina swallovi McChesnev, Desc. New Pal. Foss., p. 57, (18G0); 



Meek, Fin. Rep. U. S. Geol. Surv. Neb., p. 201, pi. ix, ff. 7a, b, (1872); 



etc. 

 Aucella hausmani Geinitz, Carb. u. Dyas. in Neb., p. 25, pi. n, f. 8, 



(1866). 



Meek's description: ''Shell rather small, nearly or quite 

 equivalve, modioliform or mytiloid, convex, or even subangu- 

 lar, along the umbonal slopes from the beak to the anterior 

 basal margin ; posterior and postero-dorsal regions, cuneate ; 

 cardinal margin nearly straight, and about half the length of 

 the shell, passing almost imperceptibly or without any angu- 

 larity into the posterior margin, which rounds down with a 

 semicircular curve to the narrowly rounded basal extremity ; 

 antero-basal margin ascending obliquely forward, more or less 

 sinuous near the middle or sometimes a little above, usually 

 swelling out into kind of lobe or protuberance above the middle 

 in front of the umbonal slope, as in Modiola. This prominence 

 sometimes extends a little beyond the beaks, and varies more 



10— vi 



