2 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



taken. Moreover, the vast collections from American waters extend 

 our knowledge of the distribution of species, both geographic and 

 bathynietric. 



The study of large series of the widely distributed or so-called cos- 

 mopoHtan species of barnacles shows that, with the exception of 

 pelagic and deep-sea forms, they may be divided into subspecies which 

 conform in distribution to the faunal provinces usually recognized 

 by workers in marine zoology, and determmed by conditions of tem- 

 perature, currents, land barriers, and depth. The definition of these 

 subspecies or local races has hitherto received but little attention in 

 this group. 



The walls of Balani have been inadequately illustrated in former 

 works. With the more intensive study of racial characters now 

 undertaken in all branches of zoology, the external characters of 

 barnacles become of importance. The extraordinary plasticity of 

 the Balanid organization makes discrimination between racial char- 

 acters and those controlled by environmental conditions a matter of 

 extreme difTiculty. Ecological and biomotrical studies of some of 

 the abimdant species are much needed in this comiection. Numer- 

 ous figures are given herein to serve as standards of comparison for 

 those who may take up such local work. 



By the copious illustration of American forms, by large-scale draw- 

 ings and photographs, their identification should be easier. Until 

 one has studied barnacles deeply enough to have standards of com- 

 parison, even the inimitable descriptions of Darwin are difficult to 

 follow, especially if one is dealmg with forms differing somewhat from 

 those described. The appeal to the eye will often reUeve the student 

 from uncertainty. ''Identifying by the pictures" may be a primi- 

 tive and superficial method, yet such short cuts are permissible in 

 case one merely needs the name of an animal which has been taken 

 as the subject of embryological, ecological, or other non-systematic 

 studies. The student must be warned, however, that many species 

 have parallel mutations, vary in the same way under similar condi- 

 tions, so that external form by itself is rarely a rehable criterion. 

 The opercular valves, the details of structure of the plates of the wall, 

 and the structure of the feet are the important characters. 



For the main facts of cirripede structure the student will naturally 

 consult one of the standard treatises on zoology, such as that of 

 Lankester. Matter relating to the group in general has therefore been 

 restricted in this work to a brief explanation of the terms used in sys- 

 tematic descriptions, in large part adapted from Darwin's monograph. 



The work is based upon the specimens contained in the United 

 States National Museum, but to complete the account of American 

 forms, some illustrations and descriptions have been drawn from 

 material in the collection of the Academy of Natural Sciences of 

 Philadelphia. By the courtesy of Mr. Samuel Henshaw, I have been 



