BRACHIOPODA. 427 



Oi-this (Dinorthis) ineedai.] 



ORTHIS (DINOBTHIS) MEEDSI W. and S. 



PLATE XXXII, FIGS. 394-'. 



1892, April 1. Orthis meedsi W. and S. American Geologist, vol. ix, p. 289. 



1892, April 9. Orthis minnesotensis SAKDESOX. Bulletin of the Minnesota Academy of -Natural 



Sciences, vol. iii, p. 332, pi. v, flgs. 14-17. 



Shell of medium size, suborbicular in outline; biconvex; anterior margin broadly 

 deflected dorsally; hinge-line about one-fourth shorter than the greatest width. Sur- 

 face marked by strongly elevated, sharply rounded striae, from forty-five to seventy 

 on each valve along the anterior margin, crossed by numerous thread-like lines of 

 growth; strise arranged in bundles of two or three, those of the ventral valve bifur- 

 cating, while on the dorsal valve increase takes place by interpolation. Exfoliated 

 specimens show two or three rows of small black spots, which may represent perfor- 

 ations in the shell substance. 



Ventral valve slightly convex, with a broad, shallow sinus; greatest elevation near 

 the apex; cardinal area of moderate width, slightly concave, somewhat elevated 

 beyond or depressed below that of the dorsal valve, perforated by a small triangular 

 delthyrium, which is bounded on each side by a thread-like elevation; beak slightly 

 incurved. Internal characters of valves as in 0. (D.) pectinella, but less sharply 

 defined. 



Dorsal valve strongly convex centrally, with the lateral-posterior areas some- 

 what concave; greatest elevation at a point one-third the length of the valve from 

 the posterior margin; cardinal area very narrow, slightly concave, with a broad 

 delthyrium, which is occupied in part by a striated cardinal process. 



This common and widely distributed species throughout Minnesota is probably 

 a lineal descendant of 0. (D.) pectinella, var. sweeneyi. At first it appears rarely with 

 that variety, and attains its distinguishing character in less than fifteen feet of shales 

 above the horizon having var. sweeneyi in abundance, and which is here no longer 

 met with. The feature distinguishing 0. (D.) meedsi and 0. pectinella, var. sweeneyi 

 is, that bifurcation and interpolation of the striae take place at a much younger 

 stage of growth in the former than in the latter, and therefore the species at 

 maturity appear quite different. In some of the localities of 0. pectinella this 

 tendency towards multiplication of the striae is very noticeable, but it invariably 

 takes place after the shell has attained more than one-half its growth. In Minne- 

 sota specimens it takes place close to the margin, and consequently after maturity 

 has been attained. In 0. (D.) meedsi the first increase in number of striae occurs 

 when the shell has attained but one-third its growth, and after this period bifurca- 

 tion and interpolation are irregular. There is, moreover, a tendency towards a 

 more convex ventral valve than in 0. (D.) pectinella, but every now and then a speci- 

 men occurs with the characteristic flattened valve of that species. 



