THE SESSILE BARNACLES. 269 



sometimes wanting and always much smaller than the orifice they 

 protect. Parietal pores, when developed, in the outer layer only. 

 Basis membranous. Each branchia is composed of two plicated 

 folds. Living attached to marine vertebrates. 



This group of genera was recognized by Leach and Gray, but 

 they included Chelonibia with the whale barnacles. Darwin elimi- 

 nated Chelon'tbia, showing that its resemblance to the others is 

 merely adaptive and superficial. We owe to him a discussion of the 

 morphology of the group so lucid that no subsequent student has 

 been able to add anything of importance. Darwin did not consider 

 the group of enough systematic importance for subfamily rank ; but 

 systematic values throughout zoology have risen, and the group is 

 now generally held to be a subfamily. Professor Gruvel has sep- 

 arated XenolaJanus as another subfamily, Xenobalanince; but while 

 this genus shows great modification, it is essentially Coronulid, with 

 many characters in common with T-ubicinella. It appears to me that 

 its affinities are obscured by segregating it in a separate subfamily. 

 The number of genera of Coronulinse has been doubled since Dar- 

 win's monograph was published. 



It is somewhat remarkable that with the exceptions of Ooronula, 

 Platylcpas, and possibly Stomatolepas, all of the genera of Coro- 

 nulina? are monotypic. 



The wall in Coromilinre is essentially poreless; or at least there 

 are no pores homologous with those of the Balaninoe, which arose, 

 as already explained, by the concrescence of lateral processes on 

 internal ribs. In many Coronulinre there are pores in the oiitcr 

 layer of the parietes, which are wholly independent in genesis from 

 those of BaJanus, and apparently were formed by the deepening and 

 closing over of external stria?. 



The body-chamber of the shell has the conic form of more normal 

 Balanida 3 only in the genus Platylepas. In other genera the cham- 

 ber is cylindric or contracted at the base, reducing the size of the 

 membranous basis. A basis of large area would doubtless be pressed 

 inward by the skin of the host. 



The genera of Coronulinae form two collateral series, indicating 

 an early split in the stock. The differences between the two series, 

 as well as nearly all those characterizing the genera, are adaptive, 

 being directly related to the modes of strengthening the shell to 

 withstand impacts. 



In the Coronulid series the parietes have no median sulcus or 

 change of sculpture, and there is never any trace of props or midribs 

 projecting into the body chamber; the oral borders of the integu- 

 ment are produced, forming a "hood." The species are singularly 

 conservative in the selection of hosts. They are known only from 

 Cetacea. 



