Larcal Stayes of l^rilobites. — Beecher. 
175- 
into a position parallel with the axial line, as in Proetus. 
The pygidium has added many new segments, and the extrem- 
ity is prolonged into a spine. 
Before proceeding further in the discussion of the protaspis, 
it is necessary to notice a number of forms of young trilobites 
which have heretofore been referred to the embryonic and lar- 
val stages, but which are now believed to belong to stages^ 
later than the protaspis. 
12 3 4 5 
Figure 1. Agnostits niuhis Beyr. ; after Barrande. 
Figure 2. Af/noxtns re.r Barr. : after Barrande. 
Figure :i. Trhiuclt-xs ornntus Sfprnb. ; after Barrande. 
Figure 4. Hi/ilnii-fphiihin s(it)irii<iides Barr. ; after Barrande. 
Figure 5. Hydroiephulux rarens Barr. ; after Barrande. 
Figure 6. (Henellus {Mesunucix) usapholdes Emmons; Ford collection ; original. 
Figure 7. Olenellus (Mesonacis) asaphoides Emmons : after Ford. 
Figure 8. Olenellus (Mesonacis) asaphoides Emmons ; after Walcott. 
Bf^sides the truly elementary forms described by Barrande 
and already noticed (^Sao /tir.siifn and Dnhiunn'tes socinUs), 
there are others whicli he referred to his second, third and 
fourth orders of development.-^ Among these Jr///o.s7*/.v may be 
taken first. The youngest forms of A<iii<>stiis muhix and J. 
rex (figures 1, 2) measure respectively^ 2 and 1.8 mm. in length, 
and the adults l.S and 15 mm. The earliest stages of the gen- 
era shown on plates VIII and IX measure less than 1 mm., 
while the adults are more than 2;") mm., with the exception of 
Pro'('ft(.s pnrn'ii.sruliis, which is seldom more tlian 10 mm. long, 
though this species has a protaspis .72 mm. in length. The 
cephalon and pygitlium of the youngest known Aijiiostns an^ 
quite separate and distinct, which is not the case with the 
typical protasjjis stage. It therefore seems ])rol)able tliat on 
account of the comparatively large size and advanced struc- 
