Fercy E. Raymond — The Pygidium of the Trilohite. 25 



life on the bottom. As had already been suggested, first, I believe, 

 by Spencer, the pygidium was probably used as a caudal fin, perhaps 

 more in the manner of the up and down motion of the tail of Sagitta 

 than in the spasmodic flaps of a lobster. That the longitudinal 

 muscles had considerable power, however, is shown by the great 

 widening of the axial lobes of many trilobites with large pygidia. 

 The appendages also show that most trilobites were swimmers as 

 well as crawlers, though none so far investigated show any particular 

 adaptation for the purpose of swimming except Triarthrus, whose 

 pygidium is small. 



The trilobite was not a powerful swimmer, but on the other hand 

 it was not in any case entirely specialized for bottom life. The 

 need for a floating type of larva was not great, since floating 

 characteristics dominate over crawling characteristics in the adults, 

 and thus such evidence as exists points to a pelagic rather than 

 a benthonic ancestor for the group. 



Backward Migration of the Eyes. 



Beecher made no attempt to explain this migration, which formed 

 so fundamental a feature in his classification. Two factors seem 

 to be involved. In many cases there was little actual change of 

 position, but the eyes of the adult are relatively further back because 

 the brim in front of them has grown outward. In others there is 

 actual backward translation, and this seems to be due to the increase 

 in size of the anterior digestive portion of the alimentary canal, 

 which bulges out the anterior end of the glabella, pushes back the 

 mouth, and doubtless, the eyes, their nerves and ganglia. This 

 enlargement of the " stomach " is probably correlated with the 

 change from a planktonic mode of life, where the food was chiefly 

 the concentrated micro-organisms, to the benthonic habitat, where 

 an omnivorous or even vegetable diet may have been adopted. 

 This change, though one of food, seems to have been so fundamental 

 that its results became impressed upon the race, and thus the position 

 of the eyes came to be of classificatory importance. 



I think it will be evident from what has been said above that 

 trilobites as unlike as Cryptolithus, Harpes, and Dionide could be 

 descended from an Agnostus-like ancestor, and that those who wish 

 to place the Agnostidae and Trinucleidae among the degenerate 

 animals must bring forward some proof for their assertions which 

 is not based upon the theory that trilobites descended from benthonic 

 annelids. For further details and a full statement of the ideas 

 expressed above, the reader is referred to a memoir by the author 

 that is shortly to be published by the Connecticut Academy of Arts 

 and Sciences. 



