276 



NEW YORK STATE MUSEUM 



in final stages. The hinge hne is straight and the cardinal angles 

 very slightly extended, subangular or even rounded ; cardinal area 

 narrow, not striated vertically, denticulate but slightly and only 

 near the delthyrium of the ventral valve. The opposite valve 

 receives this denticulate edge in a narrow crenulated groove. The 

 muscle scar of the ventral valve is short and broadly flabellate with 

 somewhat thickened and elevated margins. The deltidium is- 

 usually but partially developed. In the brachial valve the cardinal 

 process is strongly bifid, the separate parts being widely separated;. 

 dental sockets shallow. 



The surface of the valves is marked in the umbonal region b)"" 

 16 to 20 sharp angular plications, simple throughout the normal 

 contour of the valves. These primary plications with those of 

 the secondary series eventually constitute over the body of the 



■Strophonella (Amphistrophia) continens 



shell a series of fine threadlike lines separated by flat spaces in* 

 which lie fascicles of lesser order, sometimes but a single series 

 consisting of six or more lines, sometimes three or more subordinate 

 series. The general expression of the surface ornament however 

 is that of fine sharply fasciculate striation. On the interior of the 

 valves the surface is highly pustulose throughout except on the 

 muscle areas, the pustules being arranged in radial rows. 



These are the usual characters of the adult shell. The young 

 of the species are readily recognized as normally convex shells 

 with sharp and strong plications and this is a condition which when 

 maintained to maturity is expressed in such species as S t r o p h e o- 

 donta arata of the Schoharie grit of New York. 



Variant i equiplicata. We find a few of these forms in 

 which the simple sharp plication of growth is not broken up into- 

 fascicles but continues sharp over the body of the shell with very 

 sparse intercalations, so that the surface conveys the expression 

 of subequal plication and not of fasciculation. Such forms are 

 at once distinguished by their exterior. The initial striae are ai 



