SW. Ford—Additional Embryonic Forms of Trilobites. 258 
and the width of the head, exclusive of the posterior spines 0°14. 
The entire surface is plain, or without any trace whatever of 
ornamentation. : 
ig. 2 represents the second and older specimen, the left- 
hand portion of which is partly restored in the drawing. The 
place of this specimen in the developmental series removes it 
a number of steps from the form just described, allying it 
much more nearly to those forms in which the metamorphoses 
are at an end. There are fourteen body-rings, and behind 
these a minute, rudely semi-circular plate (the pygidium), 
which I believe to have been the source of all the body- 
angles 0 24. 
n fig. 8, which is an outline representation of fig. 5 of my 
former article, the third pleuron forms, with the others, a regular 
Series, the interocular spines have disappeared, the head has 
assumed the form which it afterward retains, and the develop- 
ment is completed. Between this form and the preceding one, 
ave a considerable numbe of others, which leave no doubt 
