90 HYALEID^. 



and ciliated surface, within the mantle cavity, on the ventral 

 side ; lingual teeth (of Hyalea ) l*ri, each with a strong recurved 

 hook (xii, 55). 



Hyalea, Lamarck, 1799. 



Etyni. — HyaVeos^ glassy. 



Syn. — Cavolina, Gioeni (not Brug.), ITSS. 



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 aeration. — A. Adams, Narr. Voy. Samarang^ ii, 522. 



GAMOPLEURA, Bellardi, 1881. Shell laterally impervious. H. 

 TauiHnensis, Sismonda. Tertiary ; Piedmont. 



DiACRiA, G-ray, 1840. (Pleuropus, Esch., 1825.) H. trisjnnosa, 

 Less, (xlii, 1, 8.) Shell tricuspidate, the terminal point long; 

 with lateral slits opening into the cervical aperture. 



Cleodora, Peron and Lesueur, 1810. 



Syn. — Clio, Linn, (part), Browne, not Miiller. 



Distr. — 12 sp. Atlantic, Mediterranean, Indian Ocean, Pacific, 

 Cape Horn. C. compressa, Ej^d. (xlii, 3). Fossil, 4 sp. 

 Miocene — ; Britain. C. infundibidum, Grag. 



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'l-l. The transverse bars of the gills, the heart, 

 and other organs are visible through the pellucid shell. 



BALANTiUM (Lcach), Gray, 1847. Shell triangular, depressed, 

 transversel}^ undulated; mouth oblong, oblique, narrow. Animal 

 similar to Cleodora. B. recitrouvi, one of the handsomest of the 

 pteropods, swims steadily, instead of flitting about in the lively 

 manner of the Hyalsea. G. inflaki, Eyd. (xlii, 4, 5). 



FLABELLULUM, BcUardi, 1871. Shell traiisversel}^ undulated 



