204 MANUAL OP THE MOLLUSCA. 



like the sea-snails, before the proper locomotive organs are developed 

 (Huxley). 



From this it would appear that while the Pteropoda present some analo- 

 gical resemblances to the Cephalopoda, and permanently represent the larval 

 stage of the sea-snails, they are developed on a type sufficiently peculiar to 

 entitle them to rank as a distinct group ; not indeed of equal value with the 

 Gasteropoda, 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 Hyalaa with Terebratula ; 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 ! 



SECTION A. THECOSOMATA, Bl.* 



Animal, furnished with an external shell ; head indistinct : foot and ten- 

 tacles 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, symmetrical. 

 Animal with two large fins, attached by a columellar muscle passing from 

 the apex of the shell to the base of the fins ; body inclosed 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 hook. 



H YALE A, Lamarck. 



Etym. Hyaleos, glassy. Syn. Cavolina, Gioeni not Brug. 

 Type, H. tridentata, fig. 107. 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 tri- 

 dentate. In H. trispinosa (Diacria, Gray) the lateral slits ojen 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 element of the foot. 



Distr. 19. sp. Atlantic, Medit. Indian 

 Ocean. 



Fossil, 5 sp. Miocene . Sicily, Tu- 

 rin, Dax. 



Fig. 107. H. tridentata. 

 * Theke a case, soma a body ; several of the genera have no shells. 



