130 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



external ribs are rude, rounded, and very irregular (pi. 31, fig. 

 Their irregularity is probably in part due to the extremely irregular 

 surface of the stone upon which the barnacle grew. The cone leans 

 toward the rostrum, which is much shorter than the carina. Where 

 the parieties are worn externally the very narrow tubes are laid open. 

 They closely resemble those of B. nubilis, but have no transverse septa 

 where exposed. The number of tubes is somewhat increased down- 

 ward by new septa dividing the tubes. The basis is nearly flat. Its 

 substance fills several cavities in the supporting rock, and in only two 

 places a few irregular pores are visible in the section exposed by 

 breaking it across. Elsewhere it is quite solid. 



The other examples recorded above, from more southern localities, 

 grew on nearly smooth, flat surfaces, perhaps planks. The cone leans 

 toward the carina, which is not much higher than the rostrum. The 

 exterior is perfectly preserved, the ribs stronger and more regular 

 than in the type. The proportions and external sculpture of the 

 compartments are very much as in a large Balanus balanus Linnaeus 

 (B. porcatus Da Costa). The parietal tubes have transverse septa 

 close to the base, whether farther up was not ascertained. The basis 

 is well provided with rounded or oblong pores in the Santa Bar- 

 bara specimen, but in places toward the periphery they are solidly 

 filled up. The Santa Eosa Island specimen also is well provided with 

 basal pores. It is not much more than half grown. In the San Diego 

 example (pi. 31, fig. 2) I found regular, rounded pores in one small 

 area of the basis; the rest of the exposure (a section across the base 

 and another near and parallel to the periphery) is quite impervious. 

 It is obvious that in this species there is a good deal of variation in the 

 degree of porosity of the basis ; and in some old individuals pores are 

 few and very local, therefore easily overlooked. The opercular valves 

 are identical in the Monterey and Santa Barbara examples, except 

 that in the latter they are less worn externally. The other individuals 

 examined lack opercular valves. 



Small examples of Balanus tintinnabulum calif ornicus grew on the 

 individuals from San Diego and Santa Eosa Island. 



This species and Balanus nubilis are the largest shallow-water 

 Balani of North America. When in good condition, B. aquila looks 

 like a very large, strongly ribbed B. ~balanus; but it differs from that 

 by the bifid sutural edges of the alse and by the opercular valves, 

 which especially the tergum have a remarkable resemblance to 

 those of B. psittacus. The inner structure of the walls is very similar 

 in B. aquila and B. nubilis, but in the former the compartments cohere 

 far less firmly and the sheath is longer. B. nubilis will break through 

 the compartments rather than part at the sutures. The internal or- 

 gans, so far as I examined them, are remarkably similar, particularly 

 the cirri, but it may be noted that the maxillae differ. In B. aquila 



