1G0 THE VOYAGE OF H.M.S. CHALLENGER. 



Observation. — This is the first instance of a species of Balanus being taken at so 

 considerable a depth as upwards of 500 fathoms. 



Acasta, Leach, 1817. 



This genus comprises those sessile Cirripedia which have six compartments, the 

 parietes and basis of which are not porous, and the basis of which is calcareous, cup- 

 formed, not elongated, and attached to sponges, or rarely to the outer layer of the stalk 

 of Isis. Darwin describes nine species as belonging to this genus, which are found all 

 over the world. In the Challenger collection I found one of these species represented 

 by a single specimen. It is — 



A casta finest rata, Darwin. 



Acasta fenestrate/,, Darwin, Balanidfe, 1854, p. 316. 



This species differs from the other species of the genus in the large membrane-covered 

 openings between the compartments above the basal cup. The carino-lateral parietes are 

 half as broad as the lateral parietes. The smoothness of the basal edges of the parietes 

 and of the edge of the cup, and the structure of the tergum, with its short and prominent 

 articular ridge and pointed spur, has not been investigated, as I did not wish to sacrifice 

 the only specimen. The form of the deep basal cup downwards ending in a blunt point, 

 slightly curved to one side, answers precisely to the description and figure of Darwin. 

 The only specimen was taken in the Philippine Archipelago. 



The specimen is 9 mm. in height, being smaller than those observed by Darwin, the 

 largest of which was 0'6 inches (15 mm.) in height. 



Station 208, January 17, 1875; lat. 11° 37' N., long. 123° 32' E. ; depth 

 18 fathoms; bottom, mud. 



Tetraclita, Schumacher, 1817. 



This genus comprises those sessile Cirripedia which have four compartments, sometimes 

 externally calcified together ; parietes are permeated by pores, generally forming several 

 rows. The basis is flat, irregular, calcareous, or membranous. The difference between 

 the genus Balanus and the present genus consists mainly in the absence of the carino- 

 lateral compartments, and in the numerous rows of parietal pores, Tetraclita rosea 

 1 icing the only species with but a single row. As the description Darwin gives of 

 this genus is very elaborate, and as our knowledge has not been considerably augmented 

 since the publication of that description, I think it is of no use to enter into details 

 a I tout it. 



