48 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



a 2 . Rostrum having alee, or when united with the rostral latera, the composite com- 

 partment has overlapping lateral borders; opercular valves as large as the orifice; 

 labrum with concave edge, not angularly notched in the middle. 



Family CHTHAMALID^, p. 290. 



Family BALANID^ Gray. 



1825. Balanidse LEACH, Zoological Journal, vol. 2, p. 209. GRAY, Annals of 



Philosophy, new ser., vol. 10, p. 104. 1 

 1854. Balaninss DARWIN, Monograph, p. 175. 



Sessile barnacles in. which the rostrum is concrescent with the 

 rostro-lateral compartments," the composite plate having radii, or 

 overlapping the lateral compartments; sometimes all the compart- 

 ments are concrescent into one piece. The labrum has a narrow 

 median notch or cleft and is never swollen or "bullate." The cirri 

 of the third pair are more like the second than the fourth in propor- 

 tions and arrangement of spines. There are no caudal appendages. 



The family Balanidse comprises the most evolved sessile barnacles, 

 understanding by this, those which have departed farthest from the 

 ancestral pedunculate forms. Their progress has been chiefly in 

 reducing the number of compartments of the wall, increasing the 

 complexity of these compartments, and in transforming the cirri of 

 the third pair to agree in form with the second instead of the fourth 

 pair. Such forms as Pyrgoma present the extreme of reduction in 

 number of wall-plates, but they are primitive in the structure of the 

 plates. It is rather difficult to decide whether the Coronulinse, 

 Tetraclita, or the porous Balani are the most evolved, but the Balani 

 have more highly modified cirri. The Verrucidre have the wall 

 highly specialized, but the rest of the organization is not far removed 

 from the pedunculate TJwracica. 



The common ancestors of Balanidre and Chthamalidse were appar- 

 ently forms having eight wall-plates, since this number is present in 

 some genera of both families. 



M. Gravel's family Tetrameridse was proposed for genera of 

 Balanidre and Chthamalidse having the compartments reduced to 

 four. The genera Tetraclita, Elminius, Creusia, Pyrgoma, and Pyr- 

 gopsis might be segregated as a subfamily Tetraclitinae, yet as the 

 group is much more closely related to Balanus than are the other 

 subfamilies, and as the compartments of the carino-lateral pair are 

 much reduced or even eliminated in some Balani, I have thought 

 the division unnecessary. 



1 Leach and Gray were the first to use the term Balanidae, but both included various incongruous genera 

 and excluded others belonging here. Gray's Pyrgomatidte and Coronulidae are now placed in Balanidas. 



2 In Chdonibia the union is not quite complete, and the sutures are often visible. 



