ATRYPA 285 



lamellose lines of growth. Pedicle valve with a small, incurved beak 

 with the foramen and delthyrium hidden except in young individuals; 

 internally the hinge-teeth are large and widely separated, and the 

 muscular impressions are sharply defined. In the brachial valve the 

 crura are long, narrow and widely divergent, the jugum consists of two 

 processes situated posteriorly at the junction of the <-,rura with the 

 primary lamelke, directed towards the center of the shell and not joined 

 at their inner extremities, the spirals have their bases subparallel with 

 the inner surface of the pedicle valve, with their apices convergent 

 towards the center of the brachial valve. 



Remarks, The genus A try pa is commonly considered to have become 

 extinct at the close of Devonian time, but it does persist into the Missis- 

 sippian where it is limited to the Kinderhook division and is one of the 

 rarest members of the fauna. 



ATRYPA INFREQUENS n. sp. 

 Plate XXXV, Figs. 1-5 



1906. Atrypa spinosa Weller, Trans. St. Louis Acad. Sci., vol. 16, p. 443, 

 pi. 6, fig. 17. 



Description. Shell small, lenticular, subcircular in outline, the hinge- 

 line very short. The dimensions of the holotype are : length of pedicle 

 valve 7 mm., length of brachial valve 6.1 mm., greatest width 7 mm., length 

 of hinge-line 3 mm., thickness 3.5 mm. 



Pedicle valve depressed-convex, the greatest depth posterior to the 

 middle, the umbo rather prominent with the surface sloping abruptly to 

 the cardinal margin, and curving with gentle convexity to the lateral and 

 anterior margins; mesial sinus absent; the beak obtusely pointed and 

 produced posteriorly beyond the hinge-line nearly in line with the plane 

 of the valve, only slightly incurved ; cardinal area small, a little concave. 



Brachial valve a little less concave than the pedicle, flattened in the 

 umbonal region and a little compressed towards the cardinal extremities, 

 the surface curving with a very gentle convexity to the lateral and an- 

 terior margins ; the beak inconspicuous, scarcely projecting beyond the 

 hinge-line. 



Each valve marked by about fifteen rounded plications, a very few of 

 which bifurcate in the umbonal region, they increase regularly in size 

 distally, becoming very coarse at the outer margin of the shell ; the pli- 

 cations are crossed by strong, regular, concentric lines of growth which 

 are raised as lamellose extensions of the shell upon the tops of the pli- 

 cations. 



Remarks. This little shell has much the aspect of a small A. spinosa 

 from the Middle Devonian, and was originally so identified. The single 



