228 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 5 



Tetrabalanus polygenus, new species 

 PI. 27, Fig. 1 : A, B, C, D, E, F, G, H, I, J 



Description: A conical barnacle with four plates. The plates form- 

 ing the shell are smooth, and the base adheres firmly to its support. 



General appearance : At first sight this appears to be a very ordinary 

 acorn barnacle, such as a smooth specimen of Balanus crenatus. There is 

 no epidermis, and the surface of the parietes may be eroded in some places, 

 so that the cross-septa in the pores are exposed. The largest specimens 

 collected are about 10 mm. in greatest diameter, and about half that in 

 height. The cover-plates are deeply sunken below the margin of the 

 orifice. 



Shell: Fig. 1 : I, J. The shell is conical, with large orifice. The radii 

 are narrow and their true shape cannot be seen unless the shell is taken 

 apart. The interior of the shell is ribbed in the lower part, and has very 

 much the appearance of Balanus crenatus, as it is attached to the basis 

 in the same manner. The lower margin of the sheath does not hang free, 

 since the space between it and the inner lamina of the shell is filled. 



Basis: The basis is thick at the edge, thin at the center; it is smooth 

 on the interior, and has fine pores. 



Scuta: Fig. 1 : G, H. The outline of the scutum is triangular, with 

 the basal margin slightly concave. The articular ridge is high and ter- 

 minates in a free point; it is half the length of the tergal margin. The 

 adductor ridge is short and low — it looks like a continuation of the 

 articular ridge, as its lower margin is nearly in line with the lower margin 

 of that ridge. The pit for the attachment of the adductor muscle is large. 

 There is no pit for the attachment of the lateral depressor muscle. The 

 exterior of the valve is dark brown except near the occludent margin 

 where there is a lighter band ; the growth ridges are clearly marked. 



Terga: Fig. 1 : E, F. Internally the articular ridge is near the scutal 

 margin; there are four well-developed crests for the attachment of the 

 depressor muscle, and these crests project a little below the basal margin 

 of the valve. Externally the valve is almost flat, and the growth ridges 

 are not prominent. The spur is wide and is near the scutal margin. 



Maxilla: Fig. 1 : A. There is no notch below the upper large pair 

 of spines ; the spines at the lower angle are longer than those at the upper 

 angle. As usual there are two rows of spines, but only one row is shown 

 in the drawing. 



Mandibles: Fig. 1 : C. Each mandible has three large teeth; below 

 these there are two small blunt teeth ; the upper and lower margins bear 

 many small spines. 



