1894.] The Classification of the Arthropoda. 135 
must consider these forms considerably removed from the 
primitive Crustacean stock, which, in the opinion of many, was 
not far removed from the modern Phyllopoda. Both types are 
well differentiated in the lower Cambrian, and no fossils as yet 
discovered serve to bridge the gap between the two. Nor does 
the little known of Trilobitan embryology throw any light ° 
upon the question. In some there is an apparent close simi- 
larity to the early stages of Limulus, but this may easily be 
explained upon the general principles of Arthropod growth. 
Thus, in Sao, as described by Barrande (’52), in which the 
resemblance to the Xiphosures is most marked, we have but 
that increase in the number of somites from a posterior budding 
zone common to most Arthropods, while in Trinucleus (Bar- 
rande) there seems to have been an acceleration in the devel- 
opment of cephalic and pygidial regions, and then, later, an 
increase in the number of thoracic segments in that manner so 
familiar in the development of the Decapoda. The resem- 
blances to Limulus all lie in the depressed body form and the 
union of the anterior somites. 
(To be continued.) 
