276 MISSISSIPPIAN BRACHIOPODA 



the valve is not differentiated at all, or an obscure sinus is present anter- 

 iorly, which, meeting the sinus of the opposite valve, produces a slight 

 emargination of the shell ; the beak pointed, closely incurved beneath that 

 of the opposite valve. Internally a small, obscure, bilobed cardinal 

 process is present at the apex of the valve, the dental sockets are formed 

 by well-defined socket plates which are united by an imperforate, concave 

 hinge-plate, supported by a median septum, anteriorly the socket plates 

 retreat from the margins of the valves and are transformed into the 

 bases of the crura which are still joined by the concave hinge-plate and 

 are connected also with the inner surfaces of the lateral slopes of the 

 valve; the median septum continues anteriorly, sometimes to near the 

 middle of the valve, after it has become free from the hinge-plate; the 

 brachidium short, not reaching the middle of the valve. 



Surface of both valves smooth except for some concentric lines of 

 growth which vary in strength and number in different individuals, some- 

 times being practically absent and again being well developed. Shell 

 structure finely punctate. 



Remarks. This species occurs in abundance in the Pella beds of Iowa 

 and elsewhere. It has usually been identified as Dielasma turgida and only 

 recently has been described as a distinct species by Girty under the name 

 Harttina indianensis. It was placed in the genus Harttina on account of 

 the median septum of its brachial valve, but its brachidium is short like 

 that of Dielasma and it cannot, therefore, be considered as Harttina, which 

 has an elongate brachidium similar to that of Cryptonella. The combina- 

 tion of the short brachidium and the hinge-plate supported by a median 

 septum has not hitherto been recognized as of generic value, but it is a 

 stage of development essentially different from that of Dielasma, present 

 in several well-defined species and is deemed to be worthy of recognition 

 as of generic value. 



The species differs from G. turgida in its more elongate form, less 

 gibbous valves and in the usual absence or obsolesence of any median 

 depression of the brachial valve. 



Horizon, Ste. Genevieve limestone and Chester Group. 



GlRTYELLA INTERMEDIA n. sp. 



Plate XXXIV, Figs. 42-47 



Description. Shell small, ovate-subpentagonal in outline, the anterior 

 margin truncate or a little emarginate, the greatest width near or a 

 little anterior to the middle. The dimensions of two nearly complete 

 specimens are : length of pedicle valve 13.8 mm. and 13.3 mm., length 

 of brachial valve 12 mm. and 11.5 mm., greatest width 9.6 mm. and 9.9 

 mm., thickness 10 mm. and 8.5 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, the surface curving strongly towards the postero-lateral margins 



