The Scallops or Fan-shells (_Pectinidæ_) are well known for their beautiful colours, sculpture, and excellent flavour. The animal has a distinct foot, which is not, however, used as a locomotive organ, but employed in spinning a byssus of attachment when required. The young Pectens dart through the water by opening and suddenly closing their valves. The species are very numerous, world-wide in their distribution, and may be found at depths from a few to _three thousand_ fathoms. EULAMELLIBRANCHIA. (Fig. 25, C.) [Sidenote: Cases 160–204.] In this order the gills have interfilamentar and interlamellar _vascular_ junctions, and there are generally two adductor muscles. The order is very extensive, comprising nearly sixty families, of which only the more important can be referred to. The _Carditidæ_ and _Astartidæ_ have strong solid shells, frequently ornamented with radiating or concentric ribbing, and usually are coated with a dark epidermis. They have the general appearance of certain _Veneridæ_; but the animal has no prolonged siphons, but merely a fringed opening in the mantle. One very remarkable species, _Thecalia concamerata_ (Case 161), has an internal cup-like process within the valves, which serves as a nursing-pouch for the young. [Sidenote: Cases 162–163.] The _Lucinidæ_ are almost invariably white shells, and may generally be recognized by the very long muscular scar in front on the inner surface of the valves. They occur in all parts of the world; and the fossil forms, which are still more numerous than those now living, have existed at every epoch from the Silurian. [Sidenote: Cases 166–177.] Of the freshwater Mussels or _Unionidæ_ more than 1200 species have been already discovered; they are found in most parts of the world, the greatest number having been described from North America. In _Unio_ the edges of the mantle are not united along the bottom and not prolonged into siphonal tubes; at the posterior end there are two openings, of which the upper or excretal orifice is simple, and the lower or branchial fringed at the edge. Two “cephalic eyes” have recently been noticed by Mr. P. Pelseneer. The foot is very large and adapted for crawling and burrowing. The sexes are distinct; and the shells of the females are somewhat more tumid than those of the males. _Margaritana margaritifera_, (Case 168) which is found in this country and in Europe, sometimes produces handsome pearls, but not equal to those obtained from the pearl-oyster of tropical seas. The hinge in this family is extremely variable, being in some instances delicate and toothless (_Anodonta_, Fig. 31, B), whilst in others it is enormously thickened and furnished with strong interlocking teeth (see Fig. 31, A). [Illustration: Fig. 31. ] [Sidenote: Case 178.]