CLASS IV GASTROPODA 



Order 1. OPISTHOBRANCHIA Milne Edwards. 



Marine, ivater-hreathing forms, either naked or shell-covered, in which the gills are 

 placed behind the heart and lie free on the back or side ; or true gills may be absent, 

 being replaced by secondary or false gills. Heart with a single auricle. 



The Opistliobranchiates, imlike the Streptoneura (Prosobranchiates), send the blood 

 into the heart from behind, instead of from the anterior side. The gills, in the form 

 of a more or less branched plume, lie on the right side, or are replaced by false gills 

 not homologous with the ctenidium, arranged either in two rows on the back, or 

 wreath-like aroiind the anus. The gills are often covered by the mantle, and some- 

 times become completely atrophied. The radula generally resembles that of the 

 Pulmonates. The body and nervous System usually exhibit bilateral symmetry. 



Three suborders are recognised in the recent fauna : (1) Nudihranchiata, in which 

 a Shell is absent, except diuing the larval stage, and the ctenidium is replaced by false 

 gills ; abundantly distributed in all seas at present, but owing to their perishable 

 nature are unknown as fossils ; (2) the Tectibranchiata, in which a mantle, shell and 

 ctenidium or true gill is developed ; and (3) the Pteropoda, dating from the Cambrian, 

 and from which the second suborder is perhaps derived. A provisional fourth sub- 

 order, the Gonularida, contains Paleozoic forms of doubtful affinities, of which part 

 are probably not Mollusca. 



Suborder B. TECTIBRANCHIATA.^ 



This group, briefly defined above, has fossil representatives as early as the 

 Paleozoic. During the Mesozoic, a few genera now extinct were very profuse. Most 

 of the Tertiary species belong to existing genera. 



Family I. Acteonidae d'Orbigny. 



Shell ovate, with exposed spire, the surface usually grooved and punctured, sometimes 

 smooth. Apertur e long, rounded below ; columella generally twisted, or with folds. 

 Operculum paucispiral. Carboniferous to Recent. 



Solidula Fischer von Waldheim {Buccinulus Adams ; Dactylus Schum.). Ovate or 

 oblong, compact, solid, with a short conic spire. Columella bearing two plications, the 

 anterior prominent and bifid, the posterior comparatively inconspicuous when the 

 Shell is entire ; between them the columella is spirally excavated. 

 A few ill-defined species from the French Eocene and Miocene, one 

 from the Australian Pliocene, and numerous Recent tropical species 

 are known. 



Tornatellaea Conrad (Fig. 1054). Differs . from Solidula and 

 Acteon in the more anterior disposition of the two columellar 

 plications, in the marked depression on the anterior portion of the pio. 1054. 

 aperture, and in the greater thickness of the shell near the outer Tornatellaea Hm- 

 border of the aperture, which is frequently crenulated. Base of Jura ^'eTef Lattdofr; 

 to Miocene ; widely distributed. Type, T. bella Conrad. Subgenus : near Bemburg. 

 Triploca Täte. Eocene ; Australia. 



Acteon Montf. {Tornatella Lam. ; Speo Risso ; Kanilla Silvert.). Oval, spirally 

 punctate-striate, with conic spire. Protoconch not very prominent ; nucleus sinistral. 



^ Literature (see also preceding bibliographies) : Cossmann, M., Essais de paleoconchologie 

 comparee, i., 1S95. —Püshn/, IL A., Monograph of Recent Tectibranchiata, in Manual of Conch- 

 ology, vols. XV., xvi., 1894-95. 



