302 BALANID.E. 



to the recent B. allium, an inhabitant of the Barrier Reef of Australia. 

 The longitudinally folded variety {b) can hardly be distinguished by ex- 

 ternal aspect, or even by the opercular valves, from B. crenatus ; but 

 when the shell is disarticulated, the porose walls and non-porose basis 

 of B. crenatus, allow of no mistake in the diagnosis of the two species. 



2. Sub-Genus — Acasta. PL 9. 



Acasta. Leach. Journal de Physique, torn, lxxxv, 1817. 



Compartments six ; parietes and basis non-porose: basis 

 calcareous, cup-formed, not elongated, attached to sponges, or 

 rarely to the bar k of Isis. 



Distribution mundane; imbedded in sponges and the sponge-like bark of 

 Isis. 



This sub-genus, in one sense, is a very natural one, inas- 

 much as all the species are closely allied in essential struc- 

 ture, in general appearance, and in habit. On the other 

 hand, in the structure of the shell, in all the characters 

 derived from the opercular valves and animal's body, 

 Acasta cannot properly be distinguished generically from 

 some species of Balanus ; thus B. navicula and cpnbiformis 

 agree in the parietes and basis. not being porose and in all 

 other essential respects, differing only in the shell being more 

 elongated in the rostro-carmal axis and in being attached 

 to Gorgonia? instead of to sponges ; yet we shall see that 

 Acasta purpurata lives imbedded in the bark of Isis, so 

 that even the habit of being imbedded in sponges fails. 

 Balanus terebratus would have been ranked as an Acasta had 

 it inhabited sponges. On the other hand, some species of 

 Balanus inhabit sponges, as is often the case with B. spon- 

 (jicola, and always with B. declivis ,• but both these species 

 are distinguished easily from Acasta, the former by its porose 

 walls and basis, and the latter by its membranous basis ; 

 it may, however, be reasonably doubted whether such dif- 

 ferences ought to be considered as even sub-generic. The 

 most important character of Acasta probably consists in the 

 anterior ramus of the fourth pair of cirri, differing slightly 



