Dr. F. Muller on Balanus armatus. 399 



v. & VI. Two of the shells from the PapilUaa seated on 

 the third empty shell ; V. has the carina, and VI. the rostrum 

 turned towards the orifice of the subjacent shell : in the former 

 the rostrum is more than four times as long as the carina, in 

 the latter the carina nearly three times as long as the rostrum ; 

 but the planes of the orifices of all these shells are nearly 

 parallel. 



VII. vShell seated on a Purjiura. 



Scuta. — The scuta are very narrow ; the occludent margin 

 is nearly or fully twice as long as the basal margin ; the ter- 

 gal margin is a little shorter than the occludent margin. The 

 apex is usually slightly cmwed upwards ; the outer surface is 

 covered with strongly projecting lines of growth, and with 

 from one to six longitudinal rows of pits, which are generally 

 very deep, and frequently of considerable width (fig. 2). In 

 twenty-eight animals taken at random, there were in the 

 lower part of the scuta : — once, two on each side ; eight times, 

 three ; eight times, four ; once five, and once six rows ; fur- 

 ther, six times, three rows upon one scutum and four on the 

 other ; twice, four on one side and five on the other ; and, lastly, 

 once, five on one side and six on the other. A single row of 

 pits was seen by me only on the two animals adhering to 

 Purpura. In the inside of the scutum there is an articular 

 ridge, not of great breadth, which reaches beyond the middle, 

 or even to the lower third of the scutum, and terminates there 

 in a rounded end or in a small point. The adductor ridge is 

 inconsiderable, and extends scarcely further downwards than 

 the articular ridge. Sometimes there is a very fine, sharp 

 longitudinal ridge between the articular and adductor ridges. 

 For the musculus depressor lateralis there exists a pit which 

 is usually narrow and deep. In the larger animals especially 

 the scuta are often of remarkable thickness. 



Terga. — These agree perfectly with the descriptions given 

 by Darwin of Balanus trigonus. The six to seven ridges for 

 the musculus depressor* never extend beyond the basal mar- 



* Darwin's statements as to tlie action of the three pairs of muscles 

 which descend fi'om the opercuhim towards the base of the shell appear 

 to me, from what I have observed, especially in Tetraclita jyorosa, not to 

 be quite correct. According- to Darwin, the operculum is opened by the 

 depressores scuti laterales ; sudden contractions of the depressores ros- 

 trales probably cause the strokes which the animal gives with the beak- 

 like apices of the terga ; by the common contraction of the three pairs 

 the operculum is held down with surprising force ; the operculum can 

 only be raised by the pressure of the body against the base. (Darwin, 

 * Balanidje,' p. 62.) 



If we attempt to cut the operculum of Tetraclita porosa out of the shell, 

 the knife will tiud a free passage everywhere except in two opposite 



