348 MANFAl OF THE MOLITJSCA. 



as a distinct group ; not indeed of equal value with tlie OasterO" 

 foda, 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 Terehratula ; but 

 they made the ventral plate of one answer to the dorsal valve of 

 the other, and the anterior cephalic orifice of the pteropodous 

 shell correspond with the 'posterior^ byssal foramen of the 

 bivalve ! 



Sectioit a. — Thecosomata, B1.* 



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 cavity. 



Family I. — Hyaleid^. 



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 ZT^/a/m) 1.1.1, each with a strong recurved 

 hook. 



Hyalea, Lamarck 



Etymology, hyaleos, glassy. 

 Synonym, Cavolina, Gioeni, not Brug. 

 TijjJe, H. tridentata. Fig. 143. PI. XIY., Fig. 32. 

 Shell globular, translucent ; dorsal plate lather flat, produced 

 into a hood ; aperture contracted, with a slit on each side ; 



posterior extremity tridentate. In 



/S ^^ -^^^^Z"^^""^ ^^ H. trispinosa [Diacria, Grray) the 



^<iA ^. -Jy lateral slits open into the cervical 



[ aperture. 



I L Animal with long appendages to the 



///*"^^^\\ mantle, passing through the lateral 



// W \\ slits of the shell ; tentacles indistinct ; 



/ v fins united by a semicircular ventral 



' F lobe, the equivalent of the posterior 



Fig. 143. H. tridentata. element of the foot. 



Distribution, 19 species. Atlantic, Mediterranean, Indian 

 <^cean. 

 Fossil, 6 species. Miocene — . Sicily, Turin, Dax. 



* Theke, a case, soma, a body ; several of the geuera have no eliells. 



