Genus Tetradeion Stebbing, 1899.

• Stebbing, 1899: 207; 1906: 157.

• Chilton, 1924: 632.

“Body short and stout, smooth, head small. Side-plates 1-4 together forming a continuous shield, the contiguous margins neatly fitting, fourth larger than first to third combined, fifth small, fitting into emargination of the fourth, sixth and seventh obsolete. Antennae 1 and 2 small. Mouthparts similar to those of Phippsia. Gnathopoda 1 and 2 similar, not subchelate, merus and carpus slightly produced, propod small. Peraeopoda 1 to 4 slender, basal joints not expanded, 5 greatly reduced, consisting of a small plate representing the basal joint. Uropoda short. Telson oval, short, entire.” —Chilton 1924.

Only one species is known.

Tetradeion crassum (Chilton), 1883. (Figs. 1-13.)

• Cyproidea ? Chilton, 1883 80, Pl 3. Fig. 1.

• Tetradeion crassum Stebbing, 1899: 207, Stebbing. 1906: 157. Chilton, 1924: 631-637. Figs. 1-20. Barnard, 1930: 329.

This species has been quite adequately described by Chilton (1924), and I intend to add only a few notes and drawings to complete his description. I have given a drawing of the whole animal since Chilton's later drawing (1924) does not accurately represent the shape of the 1st sideplate and slightly falsifies the shape of the 5th, at least as indicated by my specimen. It is just possible that Chilton's figures are based on a different species from mine, as also suggested by the 5th peraeopod and one or two other minor differences. However, in my opinion there is not sufficient evidence to distinguish two species and the differences noted may equally well be due to differences in age or sex of the specimens.

Part of the original type specimen is present in the Chilton Collection. It agrees reasonably well with the specimen that I have, and with others in the Collection except for a set of slides of Lyttelton material (Tray 114, 12-14) which appear to be those used for the 1924 drawings.

The 4th and 5th segments of the peduncle of the second antenna in my specimen are subequal, longer than the 3rd. The upper lip is asymmetrical. The 1st maxilla has 4 setae on the palp; the 2nd maxilla has about 30 setules on the inner plate, the outermost ones are strong and fine-combed. The palp setules have a double barb distally, the lower barb strong, the upper one hardly deserving the term but with a ring of very fine bristle-hooks In front view the effect is that of two small swellings separated by a constriction, the smaller upper one with its ring of bristle-hooks (Fig. 3).

In the first gnathopods, the posterior margin of the propod is not as strongly spined as figured by Chilton (1924), but is much more spined than the type which has only 5 or 6 spines. The sideplate is convex only on one margin, as shown in Chilton's Fig. 9, not as in his Fig. 1. The surface of the sideplate in this and other segments is strongly corrugated; this is barely apparent in one of Chilton's specimens, but not in any of the others. However, this is probably a matter of refractive indices of the various mounting media.