Remarks on the Genus Paradoxides of Brongniart. 343 
it is maintained by many, that an indispensable condition to the es- 
tablishment of a genus, is that some species be at the same time ex- 
hibited as typical of the whole group. When this can be done, its 
expediency and propriety is exceedingly obvious; but when we con- 
sider our imperfect views of organized beings, taken as a whole,— 
and more especially our limited knowledge of fossilized bodies, a 
knowledge, however, which is almost daily increasing ; it is manifest 
that species now at the head of a genus, will, in the progress of dis- 
covery, more naturally arrange themselves in some other position in 
the group. But when an author himself regards a particular species 
as the type of his genus, we must, I suppose, retain his generic name 
for that type, whatever may become of the other species which our 
imperfect knowledge may have led us afterwards to group with it. 
Governed by the preceding principles, I would suggest the pro- 
priety of limiting the genus Paradoxides of Brongniart to those fossil 
remains only, which are allied in their structure to the P. tessini and 
P. spinulosus, and which must be regarded a typical species. On 
the same ground I would urge the necessity of retaining the genus 
Triarthrus, which was proposed in 1832. A slight comparison of 
some of the essential characters of the two genera, will prove the 
importance of separating them. 
PaRapDOXIDEs. TRIARTHRUS. 
Body—-depressed—-not contractile.| Body—elevated—contractile. 
Costal arches—with filamentous| Costal arches—with no prolonga- 
or spinous prolongations. tions whatever. 
Lateral lobes—wider than the| Lateral lobes—not wider _ the 
middle lobe. middle lobe. 
For these reasons, and some others which might be specified, I 
conceive that the genus Triarthrus ought not to be merged in that of 
Paradoxides. 
The following may be considered as the generic characters of 
the Triarthrus. 
Body slightly convex—contractile. 
Buckler—with transverse furrows or folds. 
Oculiferous tubercles—none. 
Abdomen, with ten or more articulations. 
Lateral lobes, not prolonged as in Paradoxides. 
Tail simple, or furnished with a membranaceous expansion. 
