54 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 
EXPLANATION OF THE PLATE. 
All the figures are magnified, with the exception of Figs. 1 and 2. 
Fra. 1. Cyclas (Spheriwm) with foot extended : a, shell ; b, foot ; all natu- 
ral size. 
Fra. 11. Respiratory tubes extended : a, valves; b, tubes ; all natural size. 
Fra. 111. Oyclas (Spherium) seen from below: a, valves; b, foot; c, posterior 
transversal membrane ; d, interior pair of gills; e, exterior pair of gills, 
Fra. Iv. Animal, valves removed in order to show the mantle: a aD c, the 
mantle: a a, dorsal part; b c, inferior part open; d e, the edges; f, anterior 
adductor muscle; g, posterior adductor; h, crescent-shaped membrane in the 
anterior angle of the mantle opening; û, membrane in the posterior angle, with 
its two apertures and & its transverse muscle; J J, crescent-shaped lips formed by 
the edge of the mantle on each of its sides; m, continuation of edges running 
up towards the dorsal region; m, portion of the heart; o, intestine running 
through the heart ; p, position and outline of the liver. 
Fic. v. The transverse membrane extended: a, upper aperture, leading to 
the gill-canal; b, inferior aperture, leading to the cavity of the mantle ; 
c, filaments encircling the apertures, 
Fra. vr. Foot and abdomen: a, foot contracted; b, ligaments holding it; 
c, mouth; d, tentacula of mouth; e, liver; f, first curve of the intestine ; 
g, portion of the intestine running through the ovaries ; h, rectum ; û, ovaries 
and the small cylinders composing them; a, B, v, nervous system ; a, anterior 
ganglion ; v, posterior ganglion. 
Fra. vir. Posterior and relative size of the gills: æ b, exterior gill; c d, in- 
terior gill; e, gill-sac. 
Fic. vin. Interior gill and gill-sac : a, foot; b, mouth ; c, tentacula of mouth ; 
d e, interior gill lifted up ; f, interior membrane of gill-sac ; y, the large crescent- 
shaped aperture ; h, posterior end of interior gill. 
Fia. Ix. Section showing the gill-sac: a, mantle; b, exterior gill with its 
small gill-canal ; d, interior gill, with its gill-sac; e, exterior membrane; f, in- 
terior membrane; y, ovaries ; h, capsules; t, young. 
Fia. x. A pair of pear-shaped capsules. 
Fra. xt. Young at different stages, considerably magnified: a, valves ; 
b, protruding foot. 
