S. W. Ford—Additional Embryonic Forms of Trilobites. 251 
might be included) ; and of the latter the most if not all of the 
British species, all of the Swedish, and the American P. Harlani. 
y two species, so closely allied as are the P. spinosus and 
P. Harlani, should yet differ in the particulars mentioned, has 
all along been looked upon as a mystery; but there can be but 
little doubt that all who have seriously contemplated the mat- 
ter, have regarded these differences as possessing a deep and 
peculiar significance. 
- The five known species of the American genus Olenellus ad- 
mit of a similar grouping, and, if we confine ourselves to the 
adult forms alone, upon the ground of thoracic differences 
equally pronounced with those obtaining in the genus Para- 
doxides. Three of them, O. Thompsoni, O. Vermontanus and O. 
Gilberti have the third pleuron conspicuously prolonged beyond 
the others; while in O. asaphoides it forms, with those preceding 
and succeeding it, a regularly graduated series. The thorax of 
the fifth species, O. Howell’, has not been observed. These dif- 
Paradoxides or Olenellus, as 
hese 
&: aphoides, enlarged five diameters. Fig. 
2.— Another specimen, representing a more advanced stage of development, en- 
larged four diameters. Fig. 3.—A still older specimen, the characters of which 
are all only those of the adult, enlarged two diameters 
terms, however, as will be seen further on, are not intended to 
be expressive of sharply defined or clearly distinct groups or 
sub-groups, but are here introduced merely for the sake of con- 
venience, 
* Thirteenth Regents’ Report on the N. Y. State Cabinet, p, 119. 
