GIRTYELLA 277 



and inflected to the cardinal extremities, the curvature more or less 

 abrupt to the antero-lateral margins ; mesial sinus well developed, rather 

 narrow, rounded or flattened in the middle, variable in depth, originating 

 posterior to the middle of the valve sometimes well up on the umbo, its 

 middle portion usually slightly produced in front in a short rounded 

 extension ; the beak prominent, strongly incurved, the foramen large, 

 subovate, encroaching wholly upon the umbo. Internally the dental 

 plates are well developed and elongate, reaching anteriorly from the beak 

 for one-fourth or more than one-fourth the length of the valve. 



Brachial valve less convex than the pedicle, the greatest convexity near 

 the middle, the surface curving abruptly to the lateral margins; the 

 mesial portion of the valve somewhat flattened posteriorly and more 

 broadly flattened or slightly depressed in a shallow mesial sinus in front, 

 near the front margin in the middle of the sinus or flattened region, is a 

 narrow, rounded fold or plication which is variable in the strength of 

 its development, when strongly developed in association with the rather 

 narrowly rounded elevations at the sides of the sinus, it gives to the 

 valve a marked triplicate appearance in front; the beak pointed and 

 incurved beneath that of the opposite valve. Internally the socket 

 plates are joined by the concave hinge-plate which is supported by a 

 median septum, the crural lamella? are not distinct from the socket plates, 

 the free descending lamella? of the brachidium being processes from the 

 anterior extremity of the hinge-plate; the median septum is elongate and 

 reaches anteriorly about two-fifths the length of the valve from the 

 beak. 



Surface of both valves marked only by concentric lines of growth which 

 are usually obsolete posteriorly and are frequently obscure anteriorly. 

 The shell structure finely punctate. 



Remarks. The more extreme specimens of this species are strikingly 

 like examples of Dielasma inflata in the triplicate anterior extremity of 

 the brachial valve, but they differ fundamentally from that species in the 

 internal characters of the rostral portion of the brachial valve, besides 

 having the convex curvature of the valves more regular than in that 

 species, the abrupt geniculation of the valve being absent. As compared 

 with other species of Girtyella this one is essentially intermtediate in char- 

 acter between G. turgida and G. brevilobata. It has the strongly convex 

 valves of G. turgida but lacks the broad, rounded sinus of the brachial 

 valve which is so characteristic a feature of that shell, and has the 

 narrow, rounded, mesial plication of the same valve more constantly 

 present and more strongly developed. In this last feature the species is 

 more nearly like G. brevilobata, but it is a shorter shell with more strongly 

 convex valves than that species. 



H orizon. Chester Group, Paint Creek formation. 



