DELTHYRIS 305 



at least in mature shells, the muscular sear is strongly defined and is 

 divided longitudinally by an angular ridge which becomes more septum- 

 like towards the beak, and which in young examples with less completely 

 solidified beaks, perhaps became a distinct median septum; the dental 

 plates are short, scarcely more than ridge-like thickenings of the shell 

 along the inner borders of the delthyrium. 



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

 or posterior to the middle, compressed towards the cardinal extremities ; 

 mesial fold but little elevated above the general surface of the valve, 

 rounded or flattened on top, sometimes with a faint; mesial line which is 

 scarcely a depressed furrow; the plications of the lateral slopes similar 

 to those of the opposite valve and alternate with them, the furrows 

 bounding the mesial fold somewhat deeper and broader than those be- 

 tween the other plications. 



In addition to the plications the surface of each valve is ornamented 

 by strong, regular, sublamellose, imbricating, concentric markings, four 

 or five of which occupy the space of one millimeter, and which are 

 fimbriate along their margins. 



Remarks. This species is characterized by the great breadth of the 

 shell, its mucronate cardinal extremities, and the strong plications 

 bounding the sinus of the pedicle valve. The punctate structure 

 which these shells should exhibit if they belong to the genus Spiriferina 

 has not been observed, and consequently the species is referred to the 

 genus Delthyris. Perhaps a more important deviation from the typical 

 structure of Spiriferina is the absence of a true median septum in the 

 pedicle valve. There is along the median line of the muscular scar a 

 distinct, broadly angular median ridge, but it can scarcely be interpreted 

 as a median septum in the mature condition of the shell at least. It 

 may have been such in the younger stages of growth, before the apical 

 portion of the valve became so solidified as it is in the mature specimens, 

 but the interior of such young examples has not been observed. 



Horizon. Fern Glen formation. 



DELTHYRIS SIMILIS n. sp. 

 Plate XXXVI, Figs. 49-53 



Description. Shell small, much wider than long, the greatest width 

 along the hinge-line, the cardinal extremities subrectangular, the lateral 

 and anterior margins regularly rounded or the anterior margin somewhat 

 truncated in the middle. The dimensions of a nearly complete internal 

 cast in chert are: length of pedicle valve 14 mm., length of brachial 

 valve 10 mm., width 19 mm., thickness 12 mm., height of cardinal area 4 

 mm., width of mesial sinus in front 6.6 mm. 



