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 Opisthobranchiates, 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) Nudzbranchiata, 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.! 
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. dOrbigny. 
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 
border of the aperture, which is frequently crenulated. Base of Jura 
T. bella, Conrad. Sub- 

Fic. 1005. 
Tornatellaea simu- : : j 
lata, Sowb. Oligo- to Miocene; widely distributed. Type 
eae genus—Triploca, Tate. Eocene; Australia. 
Actaeon, Montf. (Tornatella, Lam. ; Speo, Risso; Kanilla, Silvert.). 
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 Paléoconchologie comparée, I., 1895. 
Pilsbry, H. A., Monograph of Recent Tectibranchiata, in Manual of Conchology, vols. XV., XVI. 
1894-95. 
