ON PARASITES. 



Or : — f Head-angles terminating in horns ; pygidium 



Pleurae curving J fuvToyved.-A. Canadensis. 



backwards. I Head-angles slightly rounded ; pygidium 



(^ smooth. — A. Hinchsii. 

 Pleurse curving f Pygidium furrowed. — A. Salli. 



forwards. \ Pygidium smooth. — A. platycephalus. 



The characters given above, and more especially those founded on 

 the grooving of the pygidium and the direction of the pleurse, may 

 be thought by some palaeontologists to be of little specific value. 

 The segment-markings on the pygidium may be deemed by these 

 observers as characteristic rather of age than of species ; but our 

 specimens of Asaphus Canadensis, for example, are quite as strongly 

 furrowed when of large as when of small dimensions ; and all the 

 perfect specimens of Asaphus platycephalus that we have examined, 

 small as well as large, present on the caudal shield an equally smooth 

 surface. The isolated caudal shields hitherto considered to belong 

 to young individuals of the latter species, should be referred, pro- 

 perly, we believe, to Asaphus Salli. Secondly, as to the direction 

 of the pleurae. As this character is more or less related to the 

 genal conformation of the head-shield, it ought certainly to be 

 regarded as one of no mean value. If two species of Asaphi, with 

 forward-curving and backward-curving pleurae respectively, be ex- 

 amined side by side, the distinction becomes most obvious. The 

 entire conformation of the pleura is affected by it. The pits or row 

 of single indentations on the pleurae of Asaphus SincJcsii, constitute, 

 moreover, a peculiar character. 



ON" PAEASITES. 



BT liTJCnJS OILLE, M.B. 



Bead before the Canadian Institute, Dec. 4dh, 1868. 



"With the powerful aids which the collateral sciences afford him, and 

 his own habits of careful observation, the modern student of natural 

 history in ranging over the domain of vitality could not fail to notice 

 this numerous and widely distributed class of organisms. Accordingly, 

 these forms of life have received a degree of attention commensurate 



