388 BALANID.E. 



other facts, that in the Balaninae the rostrum is composed 

 of the three anterior compartments, which we see in the 

 Chthamalinae, indissolubly united together; hence in Chelo- 

 nobia the middle one of the three partially-blended com- 

 partments is properly the rostrum, and those on the sides 

 the rostro-lateral compartments. 



Basis. — The basal membrane extends under the thick 

 walls to the outside. I was not able to make out the 

 whole cementing apparatus. The main trunk is remark- 

 able from its small diameter, (//in fig. 2, PI. 28), and from 

 the distance at which the cement-glands stand apart. Each 

 gland gives rise to a pair of cement-ducts, which tend to run 

 in parallel groups ; these ducts repeatedly bifurcate, occa- 

 sionally inosculate, and decrease in diameter ; they debouch 

 and allow the cement to escape at numerous points, placed 

 at irregular distances, on the edges of each new slip of the 

 basal membrane. 



Opercular Valves. — These are elongated; they do not 

 fill the orifice of the shell ; they are attached by a strong 

 opercular membrane a little way down the sheath. The 

 opercular membrane is generally double, for the last-formed 

 membrane is not immediately moulted as soon as a new one 

 is formed, as generally happens in Balanus. Externally, the 

 valves are marked by rather rugged, broad zones of growth. 

 The Scuta are elongated in the line of the orifice of the 

 sack; the occludent margin (PI. 14, fig. 1 b) is much in- 

 flected, and generally sinuous ; along this inflected portion 

 a distinct square-edged ridge runs, which widens from the 

 apex downwards. The depression for the adductor muscle 

 is very deep : there are no pits or crests for the other muscles. 

 When the scutum is thoroughly cleaned and all the mem- 

 brane removed by caustic potash, the tergal margin is marked 

 by a slight articular ridge and furrow. This articular ridge 

 is very remarkable from supporting a prominent, flattened 

 crest (fig. 1 b), composed of hard, yellow, horny membrane, 

 which overlaps the inner surface of the tergum, and exactly 

 corresponds, in shape and purpose, with the calcareous arti- 

 cular ridge, when best developed in other genera. Beneath 

 the basal and generally slightly sinuous exterior margin of 

 the valve, that is, the margin to which the opercular mem- 



