292 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



Structurally, the Chthamalidse stand between the Balanidio and 

 the pedunculate cirripedes, but the most primitive members now 

 existing of the two families are as unlike as the highest in essential 

 structure. In fact, the nearest approximation is found in some of the 

 more evolved members of each series, such as Pachylasma and Ba- 

 lanus forms which have been similarly remodeled. The common 

 ancestor of the two families may be presumed to be a Mesozoic genus 

 approximating Catophragiwts or Octomeris in structure of the wall. 



Chthamalidic are very rare as fossils, but it is evident that they 

 arose well down in the Mesozoic. Chthamalus darwini Bosquet of the 

 upper cretaceous is a typical species, so far as the walls are concerned, 

 and Ilexelasma appears in a gigantic species in the Miocene. As 

 neither of these can be considered primitive, it is obvious that the 

 family has had a long history. Indeed, it is probably now decadent. 

 Characteristics suggesting this view are the strikingly discontinuous 

 distribution of the species of most of the genera ; the small number of 

 species and their strong differentiation. These peculiarities are most 

 marked in the most primitive and presumably oldest genera. Chtha- 

 malus, a relatively evolved genus, is the only one which is generally 

 distributed. The others have the appearance of disconnected frag- 

 ments lingering like Limulus, the Dipnoi, or the struthious birds, 

 from an earlier time of wider distribution. As yet there is no palaeon- 

 tological evidence for this view. 



In the stations chiefly occupied by Balanidse that is, from low- 

 water mark to the edge of the continental shelf, at about 100 

 fathoms the Chihamalicbe seem to have been almost crowded out 

 of the race. Chthamalus^ the only prolific and generally distributed 

 genus, mainly inhabits a higher zone on the shore than Balanus. 

 Pachylasma. and the closely related Hexelasma are mainly inhabitants 

 of deep water, where Balanus is rare. Other genera have very few 

 species. Only two species of Octomeris, two of ChamoBsipho^ and 

 two of Catophragmus all very local and most of them rare inhabit 

 the shore zone, where they compete with Balanus and, more espe- 

 cially, Tetraclita. 



We owe the establishment of this family solely to the taxonomic 

 genius of Darwin, who first brought the genera together and dem- 

 onstrated their relationships. I have examined and dissected many 

 more species, I suppose, than anyone else, and I find all of the evi- 

 dence supports Darwin's views. I have not found any facts favoring 

 Professor Gravel's distribution of the genera into the three families 

 Octomeridse, Hexameridse, and Tetrameridae, among Balanid genera. 



KEY TO GENERA OF CHTHAMALID^E. 



a\ Four compartments, the carina and rostrum having alse ; sutures more or less 

 obliterated Chamcecipho* 



genus is not contained in the United States National Museum. 



