48 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



d^. Roatium having alte, or when united willi Ihe rostral lalera, the composite com- 

 partment has overlapping lateral borders; opercular valves as large as t.he orifice; 

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



Family CHTHAMALID^, p. 290. 



Family BALANID^ Gray. 



1825. Balanidx Leach, Zoological Journal, vol. 2, p. 209. — Gray, Annala of 



Philosophy, n%w ser., vol. 10, p. 10-1.' 

 1854. Balaninse 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 hke the second than the fourth m propor- 

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



The family Balanidse comprises the most evolved sessile barnacles, 

 miderstanding 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 f onn 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 VerrucidoB have the wall 

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

 from the pedunculate Thoracica. 



The common ancestors of Balanidas and Chthamalidse were appar- 

 ently forms having eight waU-plates, since this nmnber is present in 

 some genera of both families. 



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

 Balanidse and Chthamalidce having the compartments reduced to 

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

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

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

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

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

 the division mmecessary. 



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

 and excluded others tjelonging hero. Gray's Pyrgomatidae and Coronulidae are now placed in Balanidae. 



2 In Clulonibia the imion is not quite complete, and the sutures are often visible. 



