ACASTA SULCATA. 311 



same valves in A. spongites, but generally the spur is broader and 

 squarer. 



This species presents some remarkable varieties : in one specimen, 

 from Sydney, the parietes were externally ribbed longitudinally (fig. 2 b), 

 the ribs being roughened with minute points, giving to the shell an ele- 

 gant appearance ; and this without doubt is the^. sulcata of Lamarck, 

 procured by Peron at Shark's Bay, lat. 25° S., on the opposite of the 

 Australian continent : some specimens from Port Lincoln, in South 

 Australia, were not ribbed, only smoothly striated in longitudinal 

 lines : although both sets of specimens had almost smooth scuta, and 

 were thus different from common specimens, yet there could be no 

 doubt, from their similarity in all other points, that they did not differ 

 specifically from them, though the latter had their scuta striated, but 

 not their parietes. Hence we see that there is no relation between the 

 striae on the parietes and on the scuta. The Port Lincoln specimens, and 

 some others, were remarkable from the radii not extending down to the 

 basal cup, a minute cleft, covered only by membrane, being thus left 

 along the sutures, low down between the parietes (fig. 2 a) ; we shall 

 see this singular structure strongly developed in A. perforata. Owing 

 apparently to these clefts, the edge of the basal cup, exhibited traces 

 of six knob-like teeth, like those characteristic of A. gla.ns. 



Cirri. — The cirri resemble those of A. spongites, with the exception 

 that the segments on the posterior pairs bear only three main pairs of 

 spines. With respect to the anterior ramus of the fourth pair of cirri, 

 the following very singular facts were observed : — in a specimen from 

 New South Wales (var. with the tergum having a narrow spur), on 

 two or three of the lower, but not on the lowest, segments, the 

 front margin was produced or developed into two or three minute, 

 thick teeth, slightly curved like hooks downwards : in other specimens 

 from New South Wales (var. with the tergum having a broad spur, and 

 inhabiting the same branch of sponge with the last-mentioned variety), 

 there was no trace of these teeth. But again, in two other specimens 

 with the tergum having a broad spur (collected by different persons, 

 near Sydney), and in another from South Australia, this structure was 

 carried to an extreme, for in these (as represented, PI. 29, fig. 2) 

 there were beautifully formed teeth on the fourteen lower segments 

 (the twelve upper being without them), and likewise on the upper seg- 

 ment of the pedicel. These teeth are graduated in size on each seg- 

 ment; they are admirably adapted for securing any prey; and, in fact, 

 they convert each segment into a mandible-like organ. On the seg- 

 ments, on which these teeth are well developed, some of the regular 

 pairs of spines are aborted. 



Diagnosis. — Finally, this species, if it be, as I believe, distinct, differs 

 from A. spo7igites only in the internal surface of the parietes being 

 more strongly ribbed and brighter coloured, — in the edge of the basal 

 cup being more plainly crenated, — in the articular ridge of the scutum 

 being of a different shape, — in the spur of the tergum being often 

 broader, — in the segments of the posterior cirri having only three 

 pairs of main spines, — and, lastly, in the occasional presence of the 

 hook-like teeth on the anterior ramus of the fourth cirrus. 



