94 PALEONTOLOGY OF NEW YORK. 



sinuate valves are distinctive features and to these must be added the peculiar 

 character of the muscular scars. This subtype is largely represented in the 

 American Carboniferous faunas and from an examination of its various species 

 we deduce the following characterization : 



Shells transverse, often elongate or ficiform ; valves biconvex, the pedicle- 

 valve with a median sinus over the pallial region, and the brachial valve with a 

 corresponding ridge ; both sinus and fold may be divided by a sharp median 

 sulcus extending from the umbones to the margins. There is frequently evi- 

 dence of a single obscure lateral fold on each side of both valves. The umbo 

 of the pedicle-valve is incurved and the deltidial area is usually concealed ; the 

 foramen, however, is exposed as a circular or ovate aperture which encroaches 

 on the substance of the valve. In the pedicle-valve the diductor muscular im- 

 pressions are very faintly defined ; the adductor and pedicle impressions are as 

 in the typical forms of Athyris. In the brachial valve the hinge-plate is highly 

 developed, its upper face being subquadrate in outline, concave on the surface, 

 the concavity deepening toward the visceral foramen which lies just beneath 

 the beak ; not infrequently the foramen is closed by secretions of testaceous 

 matter. The posterior flanges of the plate pass beyond the hinge-line and into 

 the umbonal cavity of the opposite valve. The anterior face of the plate is 

 erect and the anterior edge somewhat trilobed, the lateral lobes bearing the 

 crural bases. The crura are straight and their attachment to the primary 

 lamellse is of the same character as in Cliothyris, etc. 

 The primary lamelljE, on the umbonal curve, are broad, 

 the loop usually situated posteriorly. The saddle of the 

 loop is often bilobed on its anterior margin, and frequently fio. k. 



, . . Loop of AthyrU Mnuclea, 



both it and the outer margins of the ribbon of the sec- Haii. .ui. Louis lime- 

 stone, (o.) 

 ondary volutions are fimbriated. 



The muscular impressions of this valve are very narrow, and subdivided into 



two pairs of elongate scars. The members of the posterior pair are divided 



by a median septum or ridge, which begins beneath, though it does not support 



the hinge-plate. Branching vascular sinuses are sometimes retained over the 



pallial region of both valves. 



