282 MISSISSTPPIAN BRACHIOPODA 



which is included in the foramen. Internally the dental lamellae are well 

 developed and of moderate length, their anterior extremities sometimes 

 curving inward towards the median line of the valve. 



Brachial valve a little more convex than the pedicle, the greatest con- 

 vexity posterior to the middle, the surface curving somewhat abruptly 

 from the umbonal region to the cardinal extremities and more gently to 

 the lateral and anterior margins ; the mesial portion of the valve flattened 

 in a similar manner to the flattening of the opposite valve and not in- 

 frequently depressed in a distinct but shallow median sinus; the beak 

 pointed, incurved beneath the base of the pseudodeltidium of the opposite 

 valve. Internally the characters of the apical portion of the valve are in 

 accord with the definition of the genus already given; the complete form 

 of the brachidium not known. 



Surface of both valves nearly smooth, only marked by concentric lines 

 of growth which are frequently rather strong and crowded towards the 

 anterior margin. The shell structure finely punctate. 



Remarks. This species is a close ally of D. compressa and perhaps 

 should not be separated from it. It is always a larger form, however, 

 with the brachial valve relatively more convex and with the median sinus 

 of the pedicle valve more conspicuously developed. No examples have 

 been observed in which the width exceeds the length, as sometimes 

 happens in the smaller species, and the concentric growth lines are more 

 strongly developed than in D. compressa. Most or all of these differential 

 characters might perhaps be accounted for in the larger size of the indi- 

 viduals of D. calhouiie^ms, but for the present, at least, the two forms will 

 be considered as distinct. In the internal characters of the brachial valve 

 upon which the genus is established, the two forms are essentially identi- 

 cal, the only difference being due to the greater compression of the valve 

 in D. compressa. 



H orizon. Hamburg oolite of the Kinderhook. 



Genus HAMBUKGIA Weller 



Description. Shell terebratuliform, with well developed dental lamelk* 

 in the pedicle valve. The brachial valve without cardinal process other 

 than the slight thickening of the shell margin at the apex of the valve, 

 the socket plates well developed, retreating from the lateral margins of 

 the valve anteriorly beyond the articulation of the valves, and connected 

 transversely by a deeply concave hinge-plate which is separated from 

 the inner surface of the valve by an exceedingly low and broad cavity; 

 upon the inner or concave side of the hinge-plate a pair of ridges orig- 

 inate towards the apex and diverge slightly while becoming stronger 

 anteriorly, finally passing into the bases of the crura; shortly in front 

 of the point of origin of the crural ridges on the hinge-plate the socket 



