CLARK: NEW BLASTOIDS AND BRACHIOPODS. 377 



GiRTYELLA WOODWORTHI, sp. nOV. 



Plate 2, fig. 1-12. 



Description. — Shell small, subpentagonal in outline, longer than 

 wide, greatest width in the middle or somewhat back of the middle. 

 The anterior margin is often concave. Both valves are moderately 

 convex. Longitudinally the pedicle-valve is the more strongly 

 arched, transversely the brachial. The pedicle-valve has a median 

 sinus which begins about six millimeters from the umbo and is low, 

 broad, and rounded. The sinus is not conspicuous in shells less than 

 nine or ten millimeters long. The brachial valve pos.sesses a still 

 more shallow, wide sinus, within which, throughout the length, there 

 is a low, rounded fold. Faint lines of growth occur on each valve. 



^ 9 ® Q 



Fig. 5. Six serial sections of the rostral portion of a specimen of Girtyella woodworlhi 

 Clark. X 2. 



Internally, the dental lamellae of the pedicle-valve are very short, 

 scarcely extending beyond the beak. In the brachial valve, the con- 

 cave hinge-plate, supported by a median septum is continued for 

 about one third or one quarter the length of the specimen. The 

 socket-plates are deflected upwards from the margins of the valve to 

 meet the hinge-plate, and are continued anteriorly to form the crural 

 processes. The median septum continues to about one third the 

 length of the valve. 



This form is more robust and more rounded along the anterior 

 margins than G. bretilobata Swallow which occurs in the Chester 

 formation of the Mississippi Valley. The latter has a well-developed 

 mesial sinus in the pedicle-valve, which is much narrower than the 

 sinus in the corresponding valve of 6'. woodivorthi. In the brachial 

 valve of G. brevilobata the mesial fold in the sinus occurs only at the 

 anterior end, while in G. woodicorihi it extends all along the sinus. 



Horizon and loccdity. — This species has been collected by Pro- 

 fessor J. B. Woodworth and the writer in the Lower Carboniferous 

 (Madison) limestone at Old Baldy, Montana. 



