182 
T)ie A)iiericau (ieoloyist. 
September, 1S93- 
Haviiii;' thus far reviewed the features of the primitive pro- 
taspis and some of the characters it acquired through earlier 
inheritance, together with the comparative age of the differ- 
ent groups of artliropods, it must be conceded, that, in inter- 
preting crustacean phylogen}" from the facts of ontogeny, the 
trilobites, so far as they show structure, are entitled to first 
l)lace. Moreover, since the appendages are quite fully known 
and from them the trilobite proves to be a most generalized 
and i)rimitive crustacean, still greater reliance can be placed 
on deductions based ujxin a study of this type. The recent 
discoveries of the antenn;e and the exact details of trilobite 
structure, together with the larval homologies here made and 
the concordance of trilobites with tlie theoretical original 
crustacean leave almost no doubt as to their true crustacean 
attinities. Woodward,^'^ from another point of view, reaches 
the same opinion by saying: "The trilobita, being certainly 
amongst the earliest forms of crustacea with which we are ac- 
quainted, cannot be removed from that class without destroy- 
ing its ancestral record/' 
VI. Kestokation of the Pkotaspis. 
At first thought, the attempt to reconstruct the ventral side 
of the trilobite protaspis may seem a little hazardous or pre- 
mature, but a careful consideration of all the data leads the 
writer to undertake this with some conlidence. 
