Cirri: in the first pair, the anterior ramus is nearly thrice as long as the posterior ramus. The second cirrus is short, with one ramus longer by three or four segments than the other ramus; the terminal segments are truncated. The third cirrus is about one third longer than the second cirrus. In the anterior ramus of the fourth pair, the regular pairs of spines are rather crowded together in the upper part of each segment, and the intermediate little spines and dorsal tufts are rather long in comparison with those of the fifth and sixth pairs of cirri: moreover, amongst the regular pairs, a few very minute and thick spines, pointing upwards, could be perceived. So that we here have the very unusual case of the fourth cirrus not exactly resembling the fifth and sixth pairs; and we shall see, in the following species, that this same anterior ramus of the fourth cirrus presents in addition another very extraordinary character. In the sixth cirrus there are on each segment four pairs of spines.
2. [ACASTA] SULCATA. Pl. [9], fig. [2 a]-[2 d].
ACASTA SULCATA. Lamarck. Animaux sans Vertèbres, 1818.
------ ---- Deshayes, in Guerin, Magasin de Zoologie, 1831, Tab. 24.
Carino-lateral parietes about one sixth of width of lateral parietes: inner surface of the parietes generally ribbed strongly: basis with the edge strongly crenated: orifice of shell rather small: tergum with the spur generally truncated and nearly half as wide as valve.
Var. (a) (fig. [2 b]): with the walls externally ribbed.
Var. (b) (fig. [2 a]): with small membrane-covered clefts between the edges of the walls, close above the basal cup.
Hab.—Sydney, Port Fairy, Moreton Bay in lat. 27° S., New South Wales; Southern Australia; and, according to Lamarck, Western Australia, in lat. 25° S. Mus. Brit., Cuming, Bowerbank, &c.