184 MONOGRAPHIA 



angular, lateral margin in the male nearly straight ; abdo- 

 men with the segments nearly equal, lateral margin 

 impressed, the first six with a black spot at the base of the 

 sutural angle, last segment minute and black ; legs thick 

 and strong ; anterior femora sub-angular ; ungues chestnut. 

 Length 2. 



I have found this species upon both the wild and tame 

 Swans (Cygnus ferus and Olor), and Mr. Thompson took it 

 from the Cygnus Bewickii in the neighbourhood of Belfast. 

 I have ventured to constitute this and the following species 

 a sub-genus, from a conviction that they possessed charac- 

 ters of equal importance with some of the other sub-genera. 

 Dr. Burmeister retains them in the sub-genus Nirmus, but 

 the bicornuted clypeus, the structure of the antenna?, the 

 proximity of the same, as well as the eyes, to the anterior 

 part of the head, the peculiar form of the latter, and also the 

 thorax, are all characters so striking, and deviating from 

 those of the rest of the Nirmi, that I trust I shall not be 

 accused of unnecessarily subdividing the group. Ornitho- 

 bius can be distinguished at one glance, whereas, although 

 Docophorus and Nirmus are principally distinguished by 

 the presence or absence of trabeculce, which at the extreme 

 of the series are admirable diagnostic signs, still I think 

 we may find some which belong to the debatable ground 

 between the two sub-genera, and that some Nirmi have 

 very minute trabeculae, while the majority have none; 

 when the former occur, it is extremely difficult to decide to 

 which sub-genus they belong. 



2. ORNITHOBIUS GONIOPLURUS. Denny. (Louse of the 



Canada Swan.) 

 Plate XXIII. Fig. 2. 



Pale yellow-white ; head with six black spots ; meta- 

 thorax acuminate behind ; abdomen elongate, the first 



