THE SESSILE BARNACLES. 



291 



of the mandible is pectinated. The cirri of the third pair resemble 

 the fourth and later pairs of cirri, or are of intermediate character 

 between the second and fourth. 



Every Balanoid barnacle in which both rostrum and carina have 

 ala3 belongs to this family; but it includes also one genus Pachy- 

 lasma, with the satellite or subgeneric groups Ilexelasma. and Bathy- 

 ~balanus in Avhich some species have the rostrolateral compartments 

 wholly concrescent with the rostrum in the adult stage, thus having 

 a wall like that of Dalanus. 



This family is more primitive than the Balanidse, and in most of 

 its departures from Balanid organization it approaches or is like the 

 LepadomorpTwt. The labrurn is like that of Verrucida? and the 

 pedunculate groups; also the mandibles and to a less extent the 

 maxilla 1 . The third pair of cirri is like the succeeding pairs, and 

 not, as in Balanidse, modified to resemble the second pair. Caudal 

 appendages are often present (never in Balanidse). The rostrolateral 

 compartments are often retained as separate plates, but in Balanidse 

 they are always concrescent with the rostrum. 



The genera differ a good deal in the degree in which old characters, 

 such as the caudal appendages, have been retained. Advance has 

 been in the simplification of the Avail by reduction of the number of 

 compartments. In C atopJiragmus all of the compartments of the 

 ancestral Balanomorpha may be presumed to be present. In Octo- 

 meris the outer whorls have been lost, leaving eight. Two phyletic 

 series lead from this point: (1) Pachylasma &n&Hexelasma, culmi- 

 nating in a hexamerous wall, and (2) Chthamalus and Chamcesipho, 

 the latter tetramerous. 



There has been parallel and of course totally independent reduc- 

 tion of wall-compartments in the two families Balaniclse and Chtha- 

 malidse, as shown in the following table, in which the leading genera 

 of both families are grouped to show the number of compartments 

 and the relationship within each series: 



