284 MISSISSIPPIAN BRACHIOPODA 



tidium except where it is filled by the incurved beak of the opposite valve. 

 Internally the dental lamellae are well developed and extend anteriorly 

 from the beak for about one-fourth the length of the valve. 



Brachial valve nearly equally or a little less convex than the pedicle, 

 the greatest convexity near or a little posterior to the middle, the surface 

 arched from the beak to the front with the convexity a little greater pos- 



XX 



FIG. 36. A series of nine cross-sections of the umbonal region of the brachial 

 valve of Hamburgia typa (X 2 1 /), showing the development of the deeply 

 concave hinge-plate, with the crural ridges upon it. 



teriorly, the lateral slopes convex, curving regularly from the median 

 line to the lateral margins, the curvature being more abrupt posteriorly ; 

 mesial portion of the valve not differentiated from the general convexity ; 

 the beak pointed, incurved beneath that of the opposite valve. Internally 

 the characters are in accordance with the generic description already 

 given. 



The surface of both valves is commonly marked by well-defined, concen- 

 tric lines of growth which vary in different individuals in number and 

 distribution. Shell structure minutely punctate. 



Remarks. This species is a common one in the fauna of the Kinderhook 

 oolite at Hamburg, Illinois, and occurs also in the Glen Park limestone of 

 Missouri. It resembles several species of Mississippian terebratuloid 

 shells in which the fold and sinus are obsolete, but perhaps agrees more 

 closely in external form with Dielasma chouteauense than with any other. 

 It may be distinguished from that species, however, by the less incurved 

 beak of the pedicle valve which is distinctly truncated, and usually by 

 the more numerous and stronger concentric lines of growth. The internal 

 characters of the rostral portion of the brachial valve are totally differ- 

 ent, however, in these two species, and they are not even members of the 

 same genus. 



Horizon. Hamburg oolite and Glen Park limestone of the Kinderhook. 



Family ATRYPID^E 

 Genus ATRYPA Dalman 



Description. Shell subcircular in outline, strongly inequivalved with 

 the brachial valve gibbous or with the valves subequally convex, the 

 hinge-line short and the cardinal extremities rounded, surface radially 

 plicate and usually marked also by more or less conspicuous, concentric, 



