(sect. a) 3 balanus psittacus. 207 



Hab. — Peru, Chile, Chiloe, Patagonia. Fossil in an ancient tertiary deposit, 

 Coquimbo ; and in a superficial, recent bed at S. Josef, in Patagonia. 



General appearance. — Shell either almost cylindrical or steeply 

 conical, generally flesh-coloured, sometimes pale pink ; surface either 

 smooth (when not disintegrated) or sometimes with the parietes dis- 

 tinctly and rather strongly ribbed, with the ribs distant from each 

 other : I have seen six or seven ribs on the rostrum alone. The orifice 

 in the most perfect specimens is nearly equilateral and hexagonal. 

 The radii generally are very broad, but occasionally quite narrow, and 

 even linear. The basis is generally deeply and irregularly cup-formed. 



Size. — This is the largest species in the family : I have seen a 

 specimen six inches in length and three and a half in diameter ; and 

 another specimen no less than nine in length, though only two and a 

 half inches in diameter. 



Scuta. — In full-sized specimens the surface is finely striated lon- 

 gitudinally, caused by the lines of growth being minutely sinuous ; 

 but in young specimens, until they attain a basal diameter of above 

 half an inch, the surface is smooth. The valve is transversely arched, 

 a line of flexure running from the apex to the basal margin, at about 

 one third of the width of the valve from the tergal margin. The basal 

 margin is curved nearly continuously, and extends nearly half-way up 

 the valve ; hence the basi-tergal corner is largely rounded off. The 

 articular ridge is but little prominent, and is notreflexed : the articular 

 furrow is very narrow. The adductor ridge consists of a sharp, much- 

 projecting plate, running down close to the basal margin, and is con- 

 fluent with the lower part of the articular ridge. This plate and the 

 inflected tergal margin of the valve, together form a large and deep 

 cavity, which extends up almost to the apex of the valve. The 

 depressor muscle is attached in the middle, at the lower, open end of 

 this cavity. 



Terga. — These are strongly beaked, the beak being from one third 

 to one fourth of the total length of the valve, including the spur : the 

 beak is very sharp, somewhat flattened, and bowed ; when young, and 

 when well preserved, it is coloured purple : it is penetrated by a fine 

 tubular cavity, occupied by a thread of corium, which extends about 

 half-way up it. The whole valve is narrow, being about thrice as long 

 as wide. The spur is also long and narrow ; it is seated at less than 

 its own width from the basi-scutal angle. The scutal margin is not 

 much inflected. The longitudinal furrow has its sides, in full-grown 

 specimens, closely folded together. The basal margin slopes down on 

 both sides to the spur. There are no crests, or only traces of them, 

 for the attachment of the depressor muscle. Internally, the spur is 

 prolonged, as a prominent ridge, upwards to the beak, and serves as an 

 articular ridge. In the middle, in the upper part (PL 2, fig. 3 d), 

 between this articular ridge and the carinal margin, there is a second 

 narrow ridge, which extends from the lower part of the beak half-way 

 down the valve, and then dies out. The space between these two 

 ridges, and the ridges themselves, are coloured purple, and consist of 



