344 MANUAL OF THE MOLLUSCA. 
can retire; gills contained in a dorsal mantle cavity: lingual 
teeth similar to Carinaria. 
Shell symmetrical, discoidal, sometimes closed by an oper- 
culum. 
ATLANTA, Lesueur. 
Type, A. Peronu, Pl. XTY., Figs. 21-23. 
Synonym, steira, Esch. 
Shell minute, glassy, oeesel and prominently keeled; 
nucleus dextrally spiral; aperture narrow, deeply notched at 
the keel; operculum ovate, pointed, lamellar, with a minute, 
apical, dextrally spiral nucleus. 
Animal 3-lobed; head large, sub-cylindrical; tentacles 
conical, with conspicuous eyes behind them; yentral fin 
flattened, fan-shaped, furnished with a small fringed sucker; 
tail pointed, operculigerous. 
Distribution, 18 species. Warmer parts of the Atlantic, 
Canary Islands. 
Sub-genus. Oxygyrus, Benson. Synonyms, Ladas, Cantraine ; 
Helico-phlegma, D’Orbigny. O. Keraudrenii, Pl. XITI., Figs. 
24, 25. Shell milky, narrowly umbilicated on both sides; 
nucleus not visible; back rounded, keeled only near the aper- 
ture ; body whorl, near the aperture, and keel cartilaginous; 
no apertural slit; operculum trigonal, lamellar. 4 species. 
Atlantic, Mediterranean. 
The Atlanta was discovered by Lamanon, who supposed it to 
be the living analogue of the Ammonite. The operculum of 
Oxygyrus (Pl. XIII., Fig. 25) is singularly like the Trigonellites 
(p. 182); that of Atlanta (Fig. 22) is the only example of a 
dextral operculum to a dextral shell (p. 207). 
PoRcELLIA, Léyéille. 
Example, P. Puzosi, Pl. XIV., Fig. 29. 
Shell discoidal, many-whorled ; whorls keeled or ‘coronated ; 
nucleus spiral; aperture with a narrow dorsal slit. 
Fossil, 10 species. Upper Silurian — Trias. Britain, Bel- 
gium. 
BELLEROPHON, Montfort. 
Example, B. bi-carinatus, Léy. Pl. XIV., Fig. 27. 
Synonym, Euphemus, M‘Coy. 
Shell symmetrically convoluted, globular, or discoidal, strong, 
few-whorled; whorls often sculptured; dorsally keeled; aper- 
ture sinuated and deeply notched on the dorsal side. 
