310 BALAN1D.E. 



2. Acasta SULCATA. PI. 9, fig. 2 a — 2 d. 



Acasta sulcata. Lamarck. Animaux sans Vertebres, 1818. 



— — Deshayes, in Guerin, Magasin de Zoologie, 183], 



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 : lergum witli the spur generally truncated 



and nearly half as wide as valve. 



Var. (a) fig. 2 b : with the ivalls externally ribbed. 

 Var. {h) fig. 2 a : with small membrane-covered clefts between the 

 edges of the ivalls, close above the basal cup. 



Bab.— Sidney, 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. 



I am almost ashamed to admit this species, so small are 

 its differences compared with A. spo?igites; yet I think that 

 it probably is a distinct form. 



In general appearance and character this species comes very near to 

 A. spongites. As in the latter, the parietes of the carino-lateral compart- 

 ments are narrow.* The orifice seems always to be smaller. Internally, 

 the parietes are generally much more strongly ribbed, and the edges of 

 the basal cup more plainly crenated. The sheath is generally coloured 

 of a brighter pink, sometimes tinged with orange. The average largest 

 specimens (from *3 to "5 of an inch in basal diameter) are a little larger 

 than the largest European specimens : I have seen one specimen from 

 Moreton Bay '4 in basal diameter, and from the basal cup being very 

 deep, actually '75 in height. The scutum has the articular ridge 

 not so prominent and not so abruptly cut off at the lower end, as in A. 

 spongites: on the other hand, the adductor ridge seems rather more 

 prominent ; but these differences are trifling. In the tergum the 

 breadth of the spur (fig. 2 c, 2 c?) varies in specimens taken out of the 

 same branch of sponge ; some can hardly be distinguished from the 



* In Mr. Cuming's collection there is a specimen, from Sidney, which I fully 

 believe to be the present species, but cannot be positively sure, as the opercular 

 valves have been lost, which is very remarkable from the walls of the carino- 

 lateral compartments being reduced to the thickness of a mere thread, not 

 one twentieth of the width of the lateral compartments; we here see the 

 structure of Acasta ttporillus prefigured. 



