THE SESSILE BARNACLES. 319 



This species is known by several hundred individuals upon a stone, 

 probably picked up on the shore. It is very remarkable for the 

 asynnnetry ol" the valves, giving it somewhat the aspect of VeT^ntca. 

 The closing apparatus is indeed like that of Verruca, since the smaller 

 pair of valves does not extend nearly to the summits of the larger 

 pair, is not visible externally, and takes no part in the closing of 

 the orifice. The summits of the larger valves close against the oppo- 

 site border of the orifice, the smaller valves then standing vertically 

 along the same border, lower down. It is doubtless on account of 

 the altered pose of the smaller valves that the depressor muscle pit 

 has moved from the face to the lower edge of the valve. 



I have found the forms of the valves very constant in a large 

 number of individuals. Having an impression at first that I was 

 dealing with a pathologic barnacle, I was led to go over a greater 

 number in detail than one would otherwise take time for. By the 

 smaller valves it seems related to C. fssus. The tergum of the larger 

 pair is more like that of C. panamerms. 



The slight asymmetry of the opercular valves in some other species 

 of Chthamalus has been noted elsewhere in this report. Darwin has 

 remarked upon the occasional asymmetry of^^^ scahroms. In G, 

 amsapoma, however, the asymmetry is much more pronounced and 

 has become a constant character. 



CHTHAMALUS PANAMENSIS, new species. 



Plate 75, figs. 2 to 2c. 



Type. — No. 2008, A.N.S.P., from Quarantine Island, Panama, on 

 Siphonaria gigas, collected by Samuel N. Rhoads, 1011. 



The barnacle is conic w^ith a small orifice, sometimes corroded and 

 dirty gray, but when well preserved, covered w^ith an olive brown 

 cuticle; varying from w^eakly folded to strongly, irregularly plicate 

 near the lower border. Interior white and nearly smooth; some of 

 the compartments pitted behind the rather long sheath. Sutures 

 distinct, simple. Radii extremely narrow or undeveloped. Diameter 

 5 to 7 mm. 



The scutum is closely, finel}'^ ridged with growth striae. Inside it 

 has a narrowly reflexed articular ridge, its edge convex, not project- 

 ing beyond the finely serrate tergal border of the valve. No adductor 

 ridge. Adductor pit rather deep; that for the lateral depressor 

 distinct. 



The tergum is irregularly oblong, widest in the middle, the scutal 

 margin straight, opposite margin arched, the apical end a little 

 narrower than the lower. Exterior finely marked with growth- 

 strise, and having a deep furrow from apex to spur, and traces of 

 one or two other very faint, short radial furrows. Articular ridge 



