Order 4. TELOTREMATA.—This group, which at the present day contains the largest number of species, includes the Lamp shells, so called from their resemblance to an ancient lamp. The valves are joined by a well-marked hinge, the peduncle passes through the peduncle valve through a foramen completed by two plates secreted by the mantle edges, and the brachial valve has attached to it a calcareous scaffolding of processes or loops for the support of the “arms.” The shells in this group are frequently ridged. Their colour is usually white, but sometimes red or yellow; deep-sea forms are generally vitreous. _Rhynchonella psittacea_ has a black shell with a pointed incurved beak; each of the arms forms a many coiled spiral and can be protruded beyond the shell; the brachial skeleton is comparatively small and simple, consisting of two separate processes. In _Terebratulina_ the brachial skeleton forms a simple loop; in _Magellania_ the loop is reflected on itself (Fig. 2). The beautiful and unique specimen of _Dyscolia wyvillii_, from 390 fathoms W. Indies, is remarkable for its size, being over two inches in length. The small vitreous specimens of _Terebratula wyvillii_ were obtained off Chili from a depth of 2160 fathoms; specimens of the same species were obtained also from a depth of 2900 fathoms in the North Pacific. TUNICATA. [Sidenote: Wall Case to left of main entrance to Shell Gallery.] The Tunicata are marine animals, the majority of which live, in their adult stage, a stationary life, fixed to the rocks or sea-bottom, but a comparatively small number are free-swimming. [Illustration: Fig. 1. _Ascidia mentula_ from the right side. _at_, atrial aperture; _br_, branchial aperture; _t_, test. [After Herdman: _Tunicata_, Encyc. Britannica.] ] They occur in the form of cartilaginous or leathery sacs, fleshy incrustations, solid fleshy masses, free-swimming, barrel-shaped animals, solitary or united into chains or hollow cylinders; or, lastly, of minute free-swimming tadpole-shaped organisms. To explain briefly the structure of a Tunicate, _Ascidia mentula_ (Fig. 1), is selected. The animal, which lives on a muddy bottom, in from five to twenty fathoms, resembles a conical sac fixed by the broader end, of grayish green colour and about 4 inches in height. At the narrower end are two orifices, one terminal—the branchial orifice or mouth, and the other a little lower—the atrial orifice: the former has eight lobes and the latter six. When the Ascidian is undisturbed, the orifices are wide open, and currents enter by the branchial and leave by the atrial orifice. On the least alarm, the orifices close, jets of water being at the same time squirted out; hence the popular name “Sea-squirts” given to these animals. [Illustration: Fig. 2. Diagrammatic section of _Ascidia_ representing the three sacs, and the branchial sac as the pharynx or throat.