GENUS QTTADRULA— QUADRULA SYMMETRICA. 143 



Shell compressed pyriform : viewed on the broader sides, with the 

 fundus widely convex, and the sides sloping or more or less inflected 

 toward the oral end, which is convex downward ; viewed on the narrower 

 side, ellipsoidal, with the fundus obtuse and the oral end roundly notched. 

 Mouth transversely oval and convex downward. Shell colorless, trans- 

 parent, composed of square plates arranged in transverse longitudinal or 

 more or less oblique rows. Sarcode colorless ; pseudopods digitate, from 

 one to three or more. 



Size. — Length 0.08 mm. to 0.14 mm. ; breadth 0.04 mm. to 0.9G mm ; 

 thickness 0.028 mm. to 0.048 mm. ; mouth from 0.02 mm. by 0.008 mm. to 

 0.032 mm. by 0.016 mm. 



Locality. — Dripping rocks with Fegatella, in Fairmount Park ; ditch 

 at the side of the Norristown railroad above Manayunk, Philadelphia; 

 sphagnum of the sphagnous swamps of Absecom, Vinelancl, and other 

 places in New Jersey. 



Quadrula symmetrica, figs. 20-25, pi. XXIV, the only representa- 

 tive of its genus, is remarkable for the peculiar construction of its shell, 

 which is compressed pyriform. Viewed on the broader surfaces, the out- 

 line is pyriform, or ovoid, with the sides sloping or more or less inflected, so 

 as to produce a neck of variable length. The oral end is transversely 

 convex. Viewed on the narrower sides, the outline of the shell forms a 

 long ellipse, tapering to the oral end, which appears notched. The mouth 

 is transversely oval and entire. 



The shell is perfectly colorless and transparent, and is composed of 

 square, structureless, chitinoid plates. These are arranged, with some gen- 

 eral degree of regularity, in transverse, more or less oblique, or longitudi- 

 nal rows. Longitudinally for the most part they successively increase in 

 size from the vicinity of the mouth toward the fundus. Frequently the 

 row surrounding the mouth is larger than the next, and those on the sum- 

 mit of the fundus are smaller than the preceding ones. The general 

 arrangement is like that of tiling with variable regularity. Mostly their 

 order is consecutive, but sometimes somewhat alternating. They are not 

 entirely disposed with the symmetry expi-essed by their name, for frequently 

 smallev plates break the regular succession of larger ones, and sometimes 

 one angle of a plate replaces that of a contiguous one. 



