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STUDIES IN EVOLUTION 



The more primitive characters of the trilobites as drawn 

 from the foregoing table may be summarized as follows: 

 (1) They are all free marine animals ; (2) the animal has a 

 definite configuration; (3) the larva is a proton auplius -like 

 form ; (4) the body and abdomen are richly segmented, and 

 the number of segments is variable ; (5) the head corresponds 

 to the typical crustacean; (6) the thorax and abdomen are 

 always distinct, the number of segments in each being vari- 

 able; (7) all segments except the anal bear paired append- 

 ages; (8) all appendages are typically phyllopodiform; and 

 (9) the coxal elements of all limbs form gnathobases, which 

 become organs of manducation on the head. 



It may be questioned by some whether the present state 

 of knowledge of the ventral structure of trilobites warrants 

 such general assertions as to details of organization. In the 

 first place, it must be granted that there is a remarkable 

 uniformity in the features of the dorsal crust, which natu- 

 rally reflects to a degree the differentiation and variation of 

 the organs and appendages of the ventral side. Further- 

 more, the actual appendages have been observed in such 

 diverse and characteristic genera as Trinudeus, Triarthrus, 

 Asaphus, Ceraurus, and Calymmene, and found to conform 

 closely to a single type, so that it seems safe to assume a like 

 agreement throughout. 



