426 BALANID.E. 



by the basal edges of the six midribs. The so-called 

 epidermis or membrane which covers the shell, is prolonged 

 in six flaps under the midribs ; and hence the proper basal 

 membrane has six deep indentations, receiving the above 

 flaps. The cementing apparatus is constructed on the same 

 simple type as in Coronula, as has been described in the 

 Introduction. 



Scuta and Terga. — These are narrow and long, stretch- 

 ing the whole length of the orifice of the shell. The two 

 valves touch each other, but are not properly articulated 

 together : their upper surfaces scale off in layers : they 

 resemble each other very closely in shape (fig. 1 c), being 

 simply oblong. The membrane covering the valves sup- 

 ports some very minute spines, i^th of an inch in length. 

 When a portion of valve is dissolved in acid it is seen 

 to be penetrated by very numerous tubuli. The opercular 

 membrane is rather thick : in P. decor ata I found it 

 occasionally double ; in this species it supports some 

 very minute spines ; it is attached about one third down 

 the sheath, of which the upper part is not marked (as 

 in Balanus) by regular concentric rings. The aperture be- 

 tween the valves, leading into the sack, has protuberant 

 lips, as in Coronula and Tubicinella. The normal five 

 bundles of muscles surrounding the sack, differently from 

 in the two genera just mentioned, do not thin out and 

 spread out downwards, but retaining their full thickness, 

 reach the basal membrane, to which they are attached. 



Mouth : the parts present no difference from those in 

 Coronula; between the outer maxillae we have the same 

 small flattened mentum-like projection ; and in the mandi- 

 bles, the same small intermediate teeth between the second 

 and third, and between the third and fourth main teeth, as in 

 that genus. The main teeth here are more plainly double 

 laterally. There are teeth on each side of the central notch 

 of the labrum. The cirri differ little from those of Coro- 

 nula, but the segments are not nearly so much flattened 

 and broad, and the second and third pairs are not so short. 

 The segments of the sixth pair support four pairs of main 

 spines, with a few minute intermediate spines, but with no 

 intermediate tuft. 



