THE SESSILE BAENACLES. 



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 — PacJiy- 

 lasma, with the satellite or subgeneric groups Ilcxelasma and Bathy- 

 halanus — in which some species have the rostrolateral compartments 

 wholly concresccnt Avitli the rostrum in the adult stage, thus having 

 a wall like that of Balanus. 



This famil}^ is more primitive than the Balanidtc, and in most of 

 its departures from Balanid organization it approaches or is like the 

 Lepado7norph(L. The labrum is like that of VerrucidsG and the 

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

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

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

 appendages are often present (never in Balanida?). The rostrolateral 

 couipartments are often retained as separate plates, but in Balanid<e 

 they are alwa^'s 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. xVdvance has 

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

 compartments. In Catophragmus all of the compartments of the 

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

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

 series lead from this point: (1) Pachylasma and II exelasma, culmi- 

 nating in a hexamerous wall, and (2) Chthamalus and Chmncesipho^ 

 the latter tetramerous. 



There has been parallel and of course totally independent reduc- 

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

 malicloe, 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: 



