body, the mouth-parts, and other appendages to Phippsia gibbosa, but in that species the fourth and fifth peraeopoda are not covered by the fourth side-plate, and the fifth peraeopod, though smaller than the fourth, has all the joints perfect.

It is evident that the genus Tetradeion must be placed in the family Stegocephalidae, coming close to Phippsia Stebbing (= Aspidopleurus Sars). It represents a further development along the same line, but has the fourth side-plate still more largely developed and concealing the sixth and seventh, and in consequence peraepod 5 is very greatly reduced.

Detailed Description.—Body smooth, broad, and greatly swollen, side-plates of the first four segments much deeper than their respective segments and strongly convex, so that the appendages of the head and peraeon and the whole of the pleon can be concealed from view when the animal is coiled up—the outline of the whole body then being ellipsoid. (Fig. 1.)

Tetradeion crassum Chilton.
Fig. 2.—First antenna.
Fig. 3.—Second antenna.
Fig. 4.—Mandible.
Fig. 5.—Lower lip, with extremity more highly magnified.

First side-plate subtriangular in outline, its posterior margin very convex; second strongly curved, narrow, only about one-fifth as wide as deep; third similar but less curved; fourth enormously expanded, forming a large convex shield with anterior margin nearly straight and the rounded posterior margin extending as far as the hinder end of third pleon segment; fifth side-plate small, forming a small plate fitting into an emargination on upper margin of fourth; sixth and seventh not developed, the whole of the corresponding appendages being covered by fourth side-plate. Head shorter than first peraeon segment; segments of peraeon subequal in length; first segment of pleon longer than second, third much shorter than second, lower margins of all three convex

and only slightly produced; fourth, fifth, and sixth pleon segments very small. (Fig. 1.)

Eyes of moderate size, rounded, black.

First antenna short and rather broad; first joint of peduncle not much longer than broad, second about three-fourths as long as first and about two-thirds as broad, produced at upper anterior angle into a subacute tooth or lobe, third joint small, about half as long as second; flagellum about two-thirds the length of peduncle, about six-jointed; first joint nearly as long as the next three and probably formed by coalescence of third or fourth joints; all joints of flagellum bearing tufts of long hairs on posterior or inner margins. (Fig. 2.)

Second antenna slender and longer than first, sharply bent backwards at end of first visible joint, which probably represents the third peduncular joint and is longer than the next two together, penultimate joint curved at base, about three-fourths as long as last joint of peduncle; flagellum six-jointed, about as long as last two joints of peduncle. (Fig. 3.)

Tetradeion crassum Chilton.
Fig. 6.—First maxilla; outer lobe seen in profile.
Fig. 7.—Second maxilla.
Fig. 8.—Maxilliped.

The mouth-parts prove to be very similar to those of Phippsia gibbosa and in both genera have probably been modified in correlation with the enormous development of the side-plates and the habits of the animal arising in connection therewith.

Mandible slender, elongated, nearly straight, cutting-edge formed of about six or seven short teeth; inner cutting-edge, spine row, molar tubercle, and palp entirely absent unless a long seta on the outer surface represents the palp. (Fig. 4.)

Lower lip slender, without inner lobes, outer lobes covered with numerous fine hairs, narrowing towards the extremity, which is produced on inner side into three or four teeth, the end one of which is much longer than the others. (Fig. 5.)

In the first maxilla the palp consists of two subequal oblong joints, the terminal one bearing a tuft of six or seven setules; outer lobe not well seen and represented in side view in fig. 6, but apparently ending as usual in several stout setules and bearing fine hairs along margins; inner lobe very large, inner margin convex and fringed with a row of about fifteen to twenty setules with fine hairs between them. (Fig. 6.)

The second maxilla with inner lobe broad, obliquely truncate at extremity, which bears about twenty long setules and a tuft of fine hairs on outer margin near distal end; outer lobe very slender and apparently jointed to a process extending outwards from base of inner lobe; it is free from setae except at the extremity, which bears six very long setules, the ends of which appear to be hooked or harbed. This outer lobe probably forms a brush for sweeping food-particles from the maxillipeds and other mouth-parts towards the mandibles. (Fig. 7.)

Tetradeion crassum Chilton.
Fig. 9.—First gnathopod.
Fig. 10.—Second gnathopod.

Maxillipeds with palp small and lobes very large; inner lobe narrow oblong, end truncate and bearing several stout setules and hooks, distal half of outer margin with thick fringe of long fine hairs; outer lobe very broad, nearly semicircular in shape, its outer margin very convex, apparently turned inwards and fringed with a row of fine hairs, inner margin straight, surface near inner distal angle bearing several stout curved setules; palp formed of three subequal joints, the second produced on inner side into an oval lobe fringed with long setules and bearing a tuft at outer distal angle,