KUTOEGINID^. 



581 



Ventral valve in young shells moderately convex, becoming more and more convex with 

 increase in size and age; in young shells the slope from the front margin to the apex is nearly 

 straight, and the apex terminates at the posterior edge of the valve above the more or less ele- 

 yated pseudo-area; in old shells the curvature from the front to the apex is nearly a semicircle, 

 and the apex terminates in a pointed beak overhanging the pseudo-area ; a mesial smus, of vary- 

 ing strength, occurs on man}^ shells, and in others it is entirety absent; the area is concave on 

 the outer parts; toward the center it becomes shghtly flattened or convex and thus forms a 

 very rudimentary pseudodeltidium; it extends beneath the apex at an angle of about 45° to 

 the plane of the margins of the valve; the pseudo-area extends about one-half the distance from 

 the apex to the plane of the valve and arches gently from the lateral extremities to the center. 



A partly exfohated shell shows four dark lines radiating forward from the umbo and 

 two near the sides that may have something to do with the muscle scars or vascular sinuses. 



FiGTTRE 50.— Kutorgina cingvlata (Billings). A, Enlargement of a portion of a transverse section of an old shell which shows its laminations and 

 the peculiar manner of growth (X30). B, C, Vertical sections of an old shell, showing the imbricated manner of growth of the outer thin 

 dart layer (X30). This outer layer is not preserved in the specimen represented by A. 

 The specunens from which these sections were made came from Locality 25a, near Swanton, Vermont (U. S. Nat. Mus. Cat. Nos. 15337f, 



15337g, and 15337h, respectively). 



I thought so in 1886 [1886b, p. 103] but am not sufficiently confident of it now to state it as a 

 settled conclusion. Fine punctse occur on the inner layer of the shell; also numerous radiating 

 lines about one-half millimeter apart. 



Dorsal valve gently convex in young shells; with increase in size the umbo becomes more 

 elevated and the apex points upward or terminates at the union of the pseudo-area and the 

 upward slope of the shell on the umbo; in some shells there is a tendency for the valve to 

 become slightly concave in the space between the margins and the elevated umbo; the area 

 of the valve is narrow and with little character; on some shells it slopes beneath the apex, 

 and on others it slopes backward, forming a low angle with the surface of the valve ; the, interior 

 of the dorsal valve shows a median septum with two central scars (h) and the anterolateral 

 scars (j) (PI. V, fig. Ih); numerous small vascular canals radiate from the central concave 

 area toward the flattened anterior and lateral half of the valve; a cast of the interior (PI. V, 



