GENUS NEBELA— NEBELA COLLARIS. 147 



considerably in the proportion of the different measurements. Viewed on 

 the broad surface, the outline is pyriform, with the oral end convex down- 

 ward. The sides slope downward to the mouth, sometimes nearly in a 

 straight line, sometimes with more or less inflection, giving rise to a neck 

 of variable length, though generally short. In the view of the narrower 

 surface, the outline is elliptical or more or less narrowly pyriform, with the 

 oral end concavely notched, and with the fundus obtuse, and sometimes 

 impressed at the sides. 



The transverse section of the shell is oval and evenly rounded at the 

 poles, but occasionally is narrowed gradually, or somewhat abruptly 

 approaching the latter. 



In some specimens, the section has a hexahedral outline, with concave 

 sides and prominent rounded angles, as seen in fig. 6, pi. XXIII. 



Sometimes the shell is impressed laterally and at the fundus, so as to 

 produce obtusely angular borders and a somewhat carinated appearance, 

 as seen in fig. 4, pi. XXII, representing a transverse section. 



Sometimes the narrower forms present at the lower third, at each lateral 

 border, a slight conical prominence, at the apex of which the shell appears 

 to be perforated by a minute pore, as seen in figs. 1, 2, 4, 7, pi XXIII. 



The mouth is transversely oval and entire, and has rounded com- 

 missures, as seen in fig. 4, pi. XXII. 



In composition, the shell is of extraordinary character, from the variety 

 in form and arrangement of its elements. Most frequently it is composed 

 of oval or circular disks, as represented in figs. 1-12, 15-17. The disks 

 usually hold no relationship in size with that of the shell : the smallest 

 specimens may have the largest disks, and the largest ones may be com- 

 posed of those of the smallest size. Sometimes the shell is almost entirely 

 composed of circular disks, sometimes of oval disks, and frequently the 

 two kinds are intermingled. Sometimes they are of pretty uniform size ; 

 at others, they are intermingled, of different sizes. Most frequently the 

 larger disks occupy the fundus and body and the smaller ones the lower 

 part or neck of the shell. Sometimes the larger disks are more or less scat- 

 tered, with some approach to uniformity, and the intervals are occupied by 

 smaller ones. Indeed, there exists almost any conceivable arrangement of 

 the round and oval, disks in the construction of the shell. 



Another variety of the shell, not uncommon, though less frequent than 



