CLASS IV GASTROPODA 



Order 1. OPISTHOBRANCHIA Milne Edwards. 



Marine, ivater-breathing forms, either naked or shell-covered, in ivhicli 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 Opisthobrancliiates, unlike the Streptoneura (Prosobranchiates), send the blood 

 into the heart from behind, instead of fi-om 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 arouiid 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) Nudibranchiata, in which 

 a shell is absent, except during 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 Conularida, 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 1. Acteonidae d'Orbigny. 



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

 smooth. Aperture long, rounded beloiv ; 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 siiire. Columella bearing two plications, the 

 anterior prominent and bifid, the jiosterior comparatively inconspicuous when the 

 shell is entire ; between them the columella is spii'ally 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 Tomatdiaea sim- 

 border of the aperture, which is frequently crenulated. Base of Jura ceTef ilattdof f^ 

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

 Triploca Tate. Eocene ; Australia. 



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

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



1 Literature (see also preceding bibliographies) : Cossmann, M., Essais de paleoconcliologie 

 comparee, i., 1895.— PiMry, II. A., Monograph of Recent Tectibranchiata, in Manual of Conch- 

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



