32 TH0RAC1CA. 



several important differences maybe detected; — on the one 

 hand, in the Balaninae, the presence of branchiae, the ex- 

 tremely complicated cementing apparatus, the difference in 

 structure between the third and succeeding pairs of cirri, 

 the large pnlpi, the notched lab rum, and the laterally double 

 teeth of the mandibles ; — and on the other hand, in the 

 Lepadidae, the presence of ovigerous fraena, caudal appen- 

 dages, bullate labium, and often prominent olfactory orifices. 

 But if the Lepadidae be compared in these several respects 

 with the other sub-family, or Chthamalinae, which cannot 

 possibly be removed out of the family of Balanidae, many of 

 these differences break down and disappear, in some or all 

 of the species. 



The Lepadidae include, as has previously been noticed, a 

 much greater range of difference than the Balanidae ; and 

 this is what might have been expected, for it is the most 

 ancient family, and extinction has done its work, separating 

 genera, which, in accordance to analogy, we may suppose 

 were once more nearly connected by intermediate forms. 

 The Lepadidae, in one sense, may be taken as the type of 

 their order ; for they have undergone less " morphological 

 differentiation;" that is, they differ the least from thelast larval 

 stage, and seem to give the most general idea of a Thoracic 

 Cirripede. On the other hand, if we mean, as some authors 

 do, by the word type, that form which, in the group in ques- 

 tion, has been most modified, and illustrates every pecu- 

 liarity of its class in the strongest manner, then we must 

 look to the Balaninae, and to its typical genus, Balanus, 

 for the most Cirripedial form. In this genus the different 

 portions of the carapace differ most, and subserve to a cer- 

 tain extent different ends, and in minute structure are 

 most complicated ; here the cementing apparatus, which 

 offers the main characteristic of the whole sub-class, is most 

 complex ; here the several pairs of cirri differ most from 

 each other in structure and action ; here the peculiar 

 branchiae (organs apparently derived from the modification 

 of another organ, itself confined to Cirripedes, viz., the ovi- 

 gerous fraena) are best developed ; here the nervous system 

 is most highly concentrated ; and, lastly, here we meet with 

 the largest and most massive species of the whole group. 



