50 CALIFORNIA ACADEMY OF SCIENCES. 



The opportunities for migration so far referred to are 

 sufficient to account for the spreading of a parasite spe- 

 cies among individuals of its host species, and for the 

 condition presented in cases like those of Docophorus lari 

 and Nirmus lineolatus common to many species of gulls, 

 and those of Trinoton luridum and Docophorus icterodes 

 common to many species of ducks : cases where the birds 

 are of gregarious habits, or where hybridization occurs. 



But of those cases of a parasite common to two or more 

 bird-species, one or more of which are Old World forms 

 and the other or others New World forms, a further ex- 

 planation is necessary. In this paper I ascribe to Mallo- 

 phagous species described from specimens taken on birds 

 of Europe or elsewhere not on the American continent 

 specimens of twenty-two different species of Mallophaga 

 taken on American birds. Examples of such occurrence 

 are Nirmus signatus and N. pileus from the American 

 Avocet (Recurvirostra afnericana) and described by Pia- 

 get and Nitzsch from specimens taken on the European 

 Avocet (Recurvirostra avocetta); Docophorus pertusus 

 from Fulica americana (America), originally described 

 from Fulica atra (Europe), and so on. In rare instances 

 we find a bird-species common to both the Old World 

 and the New World: certain birds of circumpolar range, 

 as Cepphus grylle, and exceptional cases like that of Puf- 

 Jinus major, come in this category. The parasites of these 

 birds will of course be common to America and to Europe. 

 But such instances are rare. A few other cases may exist 

 in which certain strong -flying maritime American and 

 European or Asiatic birds may meet occasionally on some 

 midoceanic island and a migration of parasites be effected. 

 Such instances, also, are exceptional. The occurrence 

 of a parasitic species common to European and American 

 birds, w r hich is not an infrequent matter (out of the sixty 



