SPIRORBIS. Lam. A genus composed of species of SERPULA, Auct. which are coiled round in a spiral disc like a snake at rest. S. nautiloides, fig. 5, is the common little white shell, found upon the shell of lobsters. SPIRULA. (_Spira_, a winding compass.) _Fam._ Lituolata, Lam. Lituacea, Bl.--_Descr._ Convolute, smooth, symmetrical, discoid, with parallel unconnected whorls, divided into numerous chambers by transverse septa; siphon continuous.--_Obs._ This pretty little shell is partly internal, only a part of it being visible when on the animal. Fig. 471. SPISULA. Gray. A genus composed of MACTRA fragilis, and other similar species, which have the ligament sub-external, marginal, not separate from the cartilage; with the posterior lateral teeth double in one valve, and single in the other. M. fragilis, fig. 80, is the species figured for Spisula in Mr. Gray's paper on the Mactradæ, in the second series of Loudon's Magazine of Natural History. We have since learned, however, that it was figured there by mistake, not having been intended for a Spisula, but belonging more properly to the genus Mactra, as defined by Mr. Gray, whose description of Spisula, is as follows:--"Shell ovate, trigonal, sub-angular at each end. Hinge and lateral teeth as in Mactra, but hinge of left tooth small. Siphonal inflexion ovate." The principal difference between Spisula and Mactra is, that the ligament is not separated from the cartilage in the former. SPONDYLUS. Auct. (_A shell-fish_, Ancients.) _Fam._ Pectenides, Lam. Sub-ostracea, Bl.--_Descr._ Inequivalve, sub-equilateral, irregularly foliaceous and spinose, auriculated, denticulated at the margins, attached by the lower and deeper valve; hinge rectilinear, with two prominent teeth in each valve, locking into corresponding cavities in the opposite valve; umbones separated by a broad, elongated, triangular disc in the lower valve; ligament contained in a groove, dividing the triangular area in the centre; muscular impressions one in each valve, sub-central, sub-orbicular. The Mediterranean, East and West Indies, and China, produce Spondyli most abundantly.--_Obs._ This genus is remarkable for the richness and beauty of the spines and foliations, which adorn the external surface of most of the species, the splendid colours by which many of them are varied, and the natural groupings formed by their attachment to each other. Fig. 177, and Frontispiece. SPORULUS. Montf. A genus of microscopic Foraminifera. SQUAMOSE. (_Squama_, a scale.) Scaly, covered with scales, as the pedicle of Pollicipes Mitellus, fig. 37*.