GENUS NEBELA— NEBELA FLABELLULUM. 153 



The transverse section of the shell is broadly oval, with the poles 

 angularly rounded, as seen in fig. 1 0. 



In the narrower lateral view of the shell it is elliptical, with the oral 

 pole tapering at the sides. The mouth is transversely oval. In the 

 broader view of the shell it is slightly convex downward, but in the narrow 

 view it appears as a concave notch. 



The shell of Nebela flabellulum has the same constitution and variety 

 of elements as in N. coUaris. It is most frequently composed of oval or 

 circular disks, as seen in figs. 8, 12, 17, or of the two together intermingled 

 in dififerent proportions. Sometimes the larger circular or oval disks are 

 more or less regularly distributed, and the intervals filled in with small 

 disks, as seen in fig. 16. 



Sometimes narrow rectangular plates in different proportions are inter- 

 mingled with the disks, and occasionally the former greatly predominate, 

 as seen in fig. 14. 



Occasionally I have found specimens in M^hich quadrate plates, like 

 those of Quadrula symmetrica, were mingled with the more usual structural 

 elements, as seen in fig. 19. 



The size and proportions of the shell o£ Nebela flabellulum vary but little, 

 compared with N. collaris. Commonly the length is less than the breadth, 

 and this is nearly twice in one direction what it is in the other. Often the 

 length and breadth are about equal, or the foi-mer measurement is a little 

 greater than the latter. Through varieties which are longer than broad, N. 

 flabellulum merges into N. collaris; but the former is sufficiently distinct and 

 prevajent in some localities to make it convenient to name it as a separate form. 

 Of the pyriform variety, or that with a more or less distinct neck, the 

 size ranges from gljth to g^oth of an inch in length, the gssth to g^*'^ ^^ ^^ ^^^^ 

 in breadth, gith to g^th of an inch in thickness, with the mouth ~fii by ^i^th 

 to ,4th by a^tli of an inch. 



The oblate spheroidal forms range from ^i^th to Jj,th of an inch long, 

 gith to jijth of an inch broad, ^-l^th to i,th of an inch thick, with the mouth 

 the same as in the former. 



The sarcode has the same conformation, kind of attachments, and con- 

 stitution as in Nebela collaris. The endosarc ordinarily contains a multi- 

 tude of yellowish and brownish food-balls, many of which often have a 

 highly oleaginous appearance. 



