90 HYALEID. 
and ciliated surface, within the mantle cavity, on the ventral 
side ; lingual teeth (of Hyalea) 1:1:1, each with a strong recurved 
hook (xii, 55). 
Hyaea, Lamarck, 1799. 
Htym.—Hyaleéos, glassy. 
Syn.—Cavolina, Gioeni (not Brug.), 1783. 
Distr.—19 sp. Atlantic. Mediterranean, Indian Ocean. H. 
tridentata, Gmel. (xlii, 1). H. quadridentata, Les. (xlii, 2). Fossil, 
10 sp. Miocene; Sicily, Turin, Dax, Azores. 
Shell globular, translucent; dorsal plate rather flat, produced 
into a hood; aperture contracted, with a slit on each side; pos- 
terior extremity tridentate. 
Animal with long appendages to the mantle, passing through 
the lateral slits of the shell; tentacles indistinct ; fins united by 
a semicircular ventral lobe, the equivalent of the posterior 
element of the foot. 
The long, loose, lateral, pallial prolongations, which the testa- 
ceous pteropods protrude from the lateral fissures of the shell, 
do not appear to be of much use in guiding or propelling, which 
functions are performed by the wide ‘alar expansions. They may 
assist, however, in extending the surface of the mantle for the 
purpose of aération.— A. Apams, Narr. Voy. Samarang, i il, 522. 
GAMOPLEURA, Bellardi, 1881. Shell laterally impervious. #. 
Taurinensis, Sismonda. Tertiary ; Piedmont. 
DIACRIA, Gray, 1840. (Pleuropus, Esch., 1825.) H. trixpinosa, 
Less. (xlii, 7, 8.) Shell tricuspidate, the terminal point long ; 
with lateral slits opening into the cervical aperture. 
CreoporA, Peron and Lesueur, 1810. 
Syn.—Clio, Linn. (part), Browne, not Muller. 
Distr.—l2 sp. Atlantic, Mediterranean, Indian Ocean, Pacific, 
Cape Horn. C. compressa, Eyd. (xlii, 3). Fossil, 4 sp. 
Miocene—; Britain. C. infundibulum, Crag. 
_ Shell pyramidal, three-sided, striated transversely ; ventral 
side flat, dorsal keeled; aperture simple, triangular, with the 
angles produced ; ; apex acute. 
Animal with tentacles obsolete; mantle processes short or 
absent ; fins ample, bilobed, united ventrally by a rounded lobe ; 
lingual ‘teeth I-11. The transverse bars of the gills, the heart, 
and other organs are visible through the pellucid ‘shell. 
BALANTIUM (Leach), Gray, 1847. Shell triangular, depressed, 
transversely undulated; mouth oblong, oblique, narrow. Animal 
similar to Cleodora. B. recurvum, one of the handsomest of the 
pteropods, swims steadily, instead ‘of flitting about in the lively 
manner of the Hyalea. C. inflata, Eyd. (xlii, 4,5): 
FLABELLULUM, Bellardi, 1871. Shell transversely undulated 
