692 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxiii. 



OBOLUS (LINGULEPIS) GREGWA Matthew. 



Lingulella gregwa Matthew, Bull. Xat. Hist. Soc, Ne\vBrun.s\vick, 1899, IV, p. 199, 



pi. I, figs. la-f. 

 Lingulella tnmiila ^Iatthew, Bull. Xat. Hist. Soc, Xew Brunswick, l!S99, ji. 200, pi. 



I, tigs. 2a-c. 



Leptobolus atavui^ Matthew, Bull. Xat. Hist. Sue, Xew Brunswick, 1S99, ji. 200, jil. 



II, figs. la-f. 



General form elono-ate ovate, with the ventral valve acuminate and 

 dorsal valve ovate-triangular in outline. The outlines of the valve.s 

 var}", as shown by a series of specimens. The convexity of the valves 

 varies with the condition of preservation. Those from the sandstone 

 are rather strongly convex, while in the shale they are very nmch 

 compressed. On the dorsal valve of most 3'oung shells there is a 

 marked and rather broad, shallow sinus extending- from the umbo to 

 the front, where it flattens out. One of the largest ventral valves has 

 a length of 21 mm. , with a width of 18 mm. A dorsal valve 16 mm. in 

 width has the same length; other examples are a little wider than long. 

 Surface of the shell marked by concentric strias and undulations of 

 growth, over which there is a series of very fine, elevated, sharph^ undu- 

 lating and inosculating lines that form a minute, irregular network 

 over the surface, very much like that of O. {Lingulella) ella. Where 

 the lines are strongly elevated the eflect is that of a minutely granu- 

 lose surface. When the thin outer la3'er of the shell is exfoliated the 

 surfaces of the various inner la3"ers is minuteh" granulose in addition 

 to the flattened, radiating strijfi and concentric lines of growth. The 

 interior surface of both valves is often marked by concentric rows of 

 strong pits or puncta?, very much as in O. {Lingulella) davlsL In 

 some specimens the lines of puncto? extend over the surface of the 

 visceral cavity so as to obscure the vascular markings and muscle scars. 

 In some examples only a few scattered puncta3 occur, while in others 

 the}^ are present over nearly the entire surface. The small shells are 

 thin, but the larger ones arc built up of a very thin outer laj^er and 

 several inner layers or lamellte that are more or less oblique to the outer 

 surface, especially over the anterior and lateral portions of the shell. 



The plane of the cardinal area of the ventral valve is nearly coinci- 

 dent near its edges with the edge of the shell. The area is long and 

 extends well forward on the cardinal slope. It is divided midwa}^ by 

 a narrow, rounded, deep pedicle furrow, and about halfwa}" between 

 the pedicle furrow and the lateral margins by an unusually well-defined 

 flexure line which is in line with the main vascular furrows of the inte- 

 rior of the valves; fine stria? of growth cross the area and arch around 

 the pedicle furrow parallel to the base of the area. There is practi- 

 cally no undercut beneath the area except near the flexure line at the 

 frontal margin of the area. The arx^a of the dorsal valve is short, 

 narrow, and crossed by fine lines of growth parallel to its base. 



i 



