Descriptions of two species of Tralobites. — 141 
characteristic. I therefore propose the following, as a statement of 
the essential characters of the genus: 
Clypeus antice curvatus: lobis lateralibus antice conniventibus : 
lobo medio sulcis transversis tribus ; sulcis sepius medio interruptis. 
Abdomen lobis tribus bene declaratis ; articulis duodecim vel plu- 
ribus. 
In addition to the above characters of the genus I would state the 
following, as possessed in common by the two species under consid- 
eration. 
The buckler is much broader than long, with the margin curved 
anteriorly and truncated posteriorly. The lateral lobes form a nar- 
row border in front of the middle lobe and are expanded behind, and 
marked with a single transverse furrow, near their posterior margin. 
The posterior sulcus of the middle lobe is continuous and parallel 
to the posterior margin of the same; the two preceding sulci, curve 
slightly backward and are interrupted near the medial line. In front 
of the sulci, there are two distant slightly oblong depressions, direct- 
ed obliquely outward and forward, which evidently mark the situation 
of the eyes. Nearly in front of each of the ocular depressions, there 
is observed on the fresh specimen, a short transverse line scarcely 
elevated; with respect to their nature I can only conjecture, that 
they were occasioned by antenne lying directly below them, and this 
appears probable, from the position of these organs in some of the 
recent Entomostraca most analogous to this genus of trilobites, as 
for instance the species of the genus Argulus.* A small rounded 
protuberance occupies the centre of the posterior border of the me- 
dial lobe of the buckler. : 
The abdomen is not distinct from the post-abdomen ; in all there 
are nineteen articulations, with a small rounded expansion beyond 
the posterior one. The middle lobe presents a longitudinal row of 
small spines, one on the centre of each articulation. Near its base 
it is wider than either of the lateral lobes; from the base it gradually 
tapers with a slight curve, to the extremity. The lateral lobes in- 
crease somewhat in width from their base and attain a maximum 
width about one third the length of the abdomen from the posterior 
extremity, where each about equals the corresponding width of the 
middle lobe; from this point it gradually diminishes to the tail. The 
following eee distinguish the species. 
* See a figure of an American species of this Buus; A. Catostomi, in Vol. xxxI. 
of this Journal. 
