TREMATODES OF AUSTRALIAN BIRDS. 255 



trematodes have been derived from common ancestors, and 

 also that their hosts have been derived from common 

 ancestors, and that the ancestors of the trematodes were 

 parasitic in those of the birds. For instance, Rolosbomum 

 hillii and H. eraticum, two closely related species of Holos- 

 tomum, are parasitic in various species of Larus. These 

 sea-gulls are apparently derived from common ancestors 

 in which the species of trematode that gave rise to H. 

 hillii and R. eraticum was parasitic. As the original Larus 

 spread over the earth till, in the course of time, it attained 

 the present very wide distribution of the genus, by the 

 acquisition of different characters it became split up into a 

 number of species. Evolutionary agencies were at the 

 same time working on the trematodes which accompanied 

 the birds, and one group eventually became separable from 

 another as a distinct species. 



The want of relationship between the hosts in the case 

 of the seven pairs in the third group, may be explained on 

 the supposition that in the one case or the other the parasite 

 has been acquired by the bird much more recently. 



INDEX OF NEW SPECIES DESCRIBED IN THIS PAPER. 



Trematode. Host. Page. 



Scaphanocephalus australis ...Haliaetus leucogaster ... 188 



Himasthla harrisoni ... ...Numenius cyanopus ... 195 



Acanthoparyphium spinulosum Charadrius dominicus ... 198 



Echinoparyphium oxyurum ...Herodias timoriensis ... 201 



E. harveyanum ... ... ...Micrceca fascinans... ... 204 



Echinochasmns tenuicollis . . . Phalacrocorax melanoleucus 206 



Patagifer acuminatus ... ...Ibis molucca ... ... 210 



P. fraternus ... ... ...Herodias timoriensis ... 213 



Plagiorchis spatulatus ... ...Anthus australis ... ... 214 



P. clelandi ...Petrochelidon ariel ... 216 



Dolichosaccus solecarius ...Phalacrocorax melanoleucus 218 



Levinseniella howensis ... ...Charadrius dominicus ... 220 



