348 MANUAL OF THE MOLLUSCA. 
as a distinct group; not indeed of equal value with the Gastero- 
poda, but with one of its orders. 
This group, the lowest of the univalve or encephalous orders, 
makes no approach towards the bivalves or acephala. Forskahl 
and Lamarck indeed compared Hyalcea with Terebratula; but 
they made the ventral plate of one answer to the dorsal valve of 
the other, and the anterior cephatic orifice of the pteropodcus 
shell correspond with the wosterior, byssal foramen of the 
bivalve ! 
Section A.—THECOSOMATA, BI.* 
Animal furnished with an external shell; head indistinct; 
foot and tentacles rudimentary, combined with the fins; mouth 
situated in a cavity formed by the union of the locomotive 
organs; respiratory organ contained within a mantle cayity. 
FAMIny I.—HYALEIDA. 
Shell straight or curved, globular or needle-shaped, sym- 
metrical. 
Animal with two large fins, attached by a columellar muscle 
passing from the apex of the shell to the base of the fins; body 
enclosed in a mantle; gill represented by a transversely plaited 
and ciliated surface, within the mantle cavity, on the ventral 
side; lingual teeth (of Hyalea) 1.1.1, each with a strong recurved 
haok. 
HYALEA, Lamarck 
Etymology, hyaléos, glassy. 
Synonym, Cayolina, Gioeni, not Brug. 
Type, H. tridentata, Fig. 148. Pl. XIV., Fig. 32. 
Shell globular, translucent; dorsal plate rather flat, produced 
into a hood; aperture contracted, with a slit on each side; 
posterior extremity tridentate. In 
H. trispinosa (Diacria, Gray) the 
lateral slits open into the cervical 
aperture. 
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 
Fig. 143. H. tridentata | element of the foot. 
Distribution, 19 species. Atlantic, Mediterranean, Indian 
Ocean. 
Fossil, 5 species. Muiocene—. Sicily, Turin, Dax. 
* Theke, a case, soma, a body ; several of the genera have no shells. 
