140 Descriptions of two species of Trilobites. 
last author, arising probably from the imperfection of his specimens, 
which appear to be of sufficient importance to require new descrip- 
tions, generic as well as specific. __ ; 
Fig. 2. 
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From the accompanying figures of these species, it is obvious that 
they do not strictly belong to the genus Paradoxides, as established 
by Brongniart ; for they are destitute of what this author gives as the 
most essential characteristic, viz. the extension of the arches of each 
of the lateral abdominal lobes, beyond the membrane above. ‘This 
character, however, is scarcely discernible in the P. gibbosus; and as 
these specimens have a strong resemblance in other respects to the 
Paradoxides, I concur with Harlan in referring them to this genus. 
This is an additional instance of the transitions*among the genera of 
trilobites, which interfere with the institution of perfect generic dis- 
tinctions. . 
Dr. Harlan observing the incongruities, modified the generic des- 
cription as follows :* ) rf 
*‘ Buckler destitute of oculiform tubercles; anterior border semi- 
circular; middle lobe marked with transverse furrows or bands. 
Abdomen composed of transverse bands or articulations continuous 
with those of the lateral lobes.” 
This generic description is evidently faulty. The anterior bor- 
der in the several species of Paradoxides is seldom semicircular, 
though generally curved and forming the segment of a circle; the 
transverse furrows, instead of extending across the middle lobe, as 
might be inferred from the description, are in general interrupted ; 
finally the character with respect to the abdomen is not sufficiently 
«Tt should be remarked that the most perfect specimen examined by Dr. H. 
presented but four abdominal articulations. 
