SUB-KINGDOM VI MOLLUSCA | 345 
intestinal canal, and anus; a closed, but partly lacunary circulation, assisted 
by a heart with one or more auricles, and containing a usually colourless 
body fluid or haemolymph ; a nervous system with at least three pairs of 
ganglia connected by commissures ; sexual reproduction by eggs and _sper- 
matozoa; audition and equilibration provided for by otocysts; respiration 
by ctenidial or secondary gills, or by the tegumentary surface, which may be 
invaginated to form a pulmonary sac; locomotion by a muscular organ called 
the foot, or by special parapodial structures; the organs normally paired, 
and protected by a sac-like integument called the mantle; and the visceral 
sac having a tendency toward torsion, so as to become usually asymmetrical. 
Sexually Mollusks are usually dioecious ; or, if monoecious, incapable of self- 
fertilisation. 
Owing to the homogeneity of the group, its division into classes has been 
attended with some differences of opinion, depending upon the point of view, 
the anatomist laying more stress upon certain groups of characters, and the 
morphologist upon others. From a general standpoint, the Mollusca are 
readily divisible into four classes, as follows :—Pelecypods, Scaphopods, 
Gastropods, and Cephalopods. The first of these is well marked off from the 
rest by the absence of a distinct head and of a radula, and the two groups 
have been contrasted as Glossophora (or Cephalophora) and Aglossa (or Lipo- 
cephala). Recent researches tend to show that the Pelecypoda form a degenerate 
group, more nearly related to the Protogastropod than is the latter to the 
Protocephalopod ; and that in any general arrangement, the Cephalopods are 
rather to be contrasted with the three other groups combined. 
M‘Coy, F., Synopsis of the Characters of the Carboniferous Limestone Fossils of Ireland. London, 
1862. 
Gabb, W. M., Palaeontology of California, vols. I., 1864, and II., 1869. 
Geinitz, H, B, Die Dyas. Leipzic, 1864. 
“Seguenza, Gi, Paleontologica Malacologica dei Terreni terziarii del Distretto di Messina (Mem. Soe. 
Ital. Sci. Nat., vols, IES); 1865-67. 
Laube, G., Die Fauna von St. Cassian (Denkschr. Wiener Akad., Bd. XXV.), 1866. 
VW "orthen, ‘A. H.,, Palaeontology of Illinois, vols. I.-VII. Springfield, Ill., 1866-83. 
Hoernes, M., Die fossilen Mollusken des Tertiarbeckens von Wien (Abhandl. k. h. geol. Reichsanstalt, 
Bd. IV.), 1870. 
Stoliczka, F., Cretaceous Fauna of Southern India (Mem. Geol. Survey of East India, vol. IL., 
Gastropoda, and III, Pelecypoda), 1868-71. 
Bellardi, L., and Sacco, F., I Molluschi terziari del Piemonte e delle Liguria, 1872-96. 
Gabb, W. M., Topography and Geology of Santo Domingo (Trans. Am. Phil. Soc., XV.), 1873. 
Sandberger, F., Die Land- und Siisswasser Conchylien der Vorwelt. Wiesbaden, 1875. 
Meek, F. B., Report on the Invertebrate, Cretaceous, and Tertiary Fossils of the Upper Missouri 
Country (U.S. Geol. Surv. Terr., vol. [X.), 1876. 
Fontannes, F., Les Mollusques pliocénes de la Vallée de Rhone et du Roussillon, 1879-83. 
Seguenza, G., Le formazioni terziarie nella provincia di Reggio, Calabria (R. Acad. dei Lincei), 1880. 
Waagen, W., The Salt Range Fossils (Mem. Geol. Surv. India; Palaeont. Indica, ser. XITI.), 
1880-87. 
White, C. A., Review of the non-marine fossil Mollusca of North America (Rept. U.S. Geol. Survey), 
1883. 
Holzapfel, J., Mollusken der Aachener Kreide (Palaeontographica, Bd. XXXIV.), 1887. 
Crossmann, M., Catalogue illustré des coquilles fossiles de Eocene des environs de Paris (Ann. 
Soc. Malac. Belg., vols. XXIII.-XXVIII.), 1888-94. 
Koenen, A. von, Das norddeutsche Unter-oligocin und seine Molluskenfauna (Abh. zur geol. 
Special-Karte von Preussen, Bd. X.), 1889-93. 
Dall, W. H., Contributions to the Tertiary Fauna of Florida, I.-III. (‘Trans. Wagner Inst. Science, 
vol. III.), 1890-95. 
Walcott, C. D., Fauna of the Lower Cambrian or Olenellus Zone (10th Ann. Rept. U.S. Geol. Surv.), 
1890. 
Crossmann, M., Essais de Paléoconchologie comparée. Paris, 1895. 
