484 MOLLUSCA SUB-KINGDOM vi 



Order 1. OPISTHOBRANCHIA. Milne-Edwards. 



Marine, water-breathing 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, unlike 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 around 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. 



Two sub-orders 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 ; and (2) Tectibranchiata, in which a mantle, shell, and 

 ctenidium or true gill is developed. To these it will be convenient to add a third 

 group, Pteropoda, which is here given nominal rank as a sub-order ; as well as a 

 fourth, Conularida, to contain Palaeozoic forms of doubtful affinities. 



Sub-Order A. TECTIBRANCHIATA. 1 



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

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

 of the Tertiary species belong to existing genera. 



Family 1. Actaeonidae. d'Orbigny. 



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

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

 Operculum paucispiral. Carboniferous to Recent. 



Solidula, F. de 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. 1005). Differs from Solidula and 

 Actaeon in the more anterior disposition of the two columellar 

 plications, in the marked depression on the anterior portion of the 

 aperture, and in the greater thickness of the shell near the outer 

 Tornatellaea 'simu- ljor d er f tne aperture, which, is frequently crenulated. Base of Jura 

 lata, Sowb. oiigo- to M.iocene ; widely distributed. Type T. bella, Conrad. Sub- 

 nwBernbw rf> genus Triploca, Tate. Eocene ; Australia. 



Actaeon, Montf. (Tornatella, Lam. ; Speo, Risso ; Kanilla, Silver!.). 

 Oval, spirally punctate-striate, with conic spire. Protoconch not very prominent ; 

 nucleus sinistral. Columella thick, with one strong, spiral, slightly oblique plication. 

 Upper Cretaceous to Recent. 



1 Literature (see also preceding bibliographies) : 

 Cossmann, M., Essais de Paleoconchologie comparee, I., 1895. 



Pilsbry, H. A., Monograph of Recent Tectibrauchiata, in Manual of Conchology, vols. XV., XVI. 

 1894-95. 



