172 HTDROBEANCHIATA. 



— Descr. Tubular, rude, irregular, attached ; lower valve cylindri- 

 cal, more or less lengthened, apparently divided into sections by 

 septa (considered by some authors as merely projecting layers 

 of growth) having one or two lateral tubes within ; upper valve 

 round, flat, fixed on the aperture of the tubular valve like an 

 operculum. — 06s. This genus is known only in a fossil state, and 

 but very imperfectly. Lamarck places it among his chambered 

 Cephalopoda, &c. De Blainville, considering it a true Bivalve, 

 enumerates it among his Eudistes. Cretaceous group. H. Cor- 

 nucopia. PI. xi. fig. 198. 



HYAL^A. Lamarck, 1799. (FyaZws, glass.) Fam. Pteropoda, 

 Lam. Thecosomata, Bl. — Descr. Globose, glassy, transparent^ 

 with a triangular opening at the upper part where the dorsal 

 portion advances beyond the ventral ; ventral portion vaulted ; 

 dorsal more flat ; lower extremity tridentate. — Obs. The singular 

 structures composing this genus were formerly taken for bivalves, 

 and named Anomia Tricuspidata, &c. They are now known to 

 belong to the class of molluscous animals, called Pteropoda, 

 from the wing-shaped organs of locomotion. A species of 

 Hyalsea occurs in Sicily in a fossil state. E-ecent species are 

 found in the Mediterranean, Atlantic, and Indian Oceans. H. 

 Tridentata. PL xii. fig. 226. 



HYALIJSrA. Schumacher, 1827. Marginella pallida. Thesaurus 

 Conchyliorum, PI. viii. Marginella, PI. Ixiv. fig. 108. 



HTALINA. Studer. Viteii^a, Drap. 



HTALIJSE. {Hyalus, glass.) Glassy, thin, transparent.— JFa;. 

 Carinaria Mediterranea, fig. 488. 



HYDATINA. Schumacher. A sub-genus of Bullidse, the shells 

 of which are thin and inflated, and have flat, visible spires. 

 B. venillum is an example of four species described in M. Adams' 

 Monograph, Part 11, Sowerby's Thesaurus. 



HYDEOBEANCHIATA. Bl. The first section of the order 

 Gasteropoda, Lam. containing Molluscs which breathe water 

 only ; divided into the families Tritoniana, Phyllidiana, Semi- 

 phyllidiana, Calyptracea, Bullaeana, and Aplysiana. 



