UTICA SLATE AND HUDSON-RIVER GROUP. 251 
The buckler of our species bears considerable resemblance to that of specimens of Olenus 
gibbosus from Sweden ; but the true Olenus, as in Paradoxides, has the facial suture 
terminating at the posterior margin of the buckler, distant from the angles, while in this 
one it terminates at the angles as in Calymene. The labrum found in the slate associated 
with this species, has also the form of that organ in Calymene senaria ; and taking these 
two facts in connection, I have preferred to refer the species to Calymene, since it is more 
nearly related to other species of that genus than to the Olen. There is still some little 
difficulty in determining the number of articulations of the thorax, since the greater number 
of specimens are imperfect ; and in a single large specimen there appear to be fourteen or 
fifteen. This fact indicates an increase in the number of thoracic rings with age, a character 
which Emmricu has given of the Olenus. The articulations of the lateral lobes are marked 
by a simple straight groove extending their entire length, and terminate in an obtuse 
point, unlike those of the true Oleni as represented by various authors. On account of these 
characters, it is impossible to refer the species satisfactorily to the Genus Olenus, though 
in some of its characters it evidently forms a link between that genus and Calymene. 
Farther investigation of this species may develop characters sufficiently distinct from 
Calymene to establish the Genus Triarthrus, which was proposed with a very imperfect 
knowledge of the entire animal. 
The most prominent character of this species, distinguishable in the segments of the 
thorax, are the tubercles or short spines which mark the centre of the axial lobe, and are 
usually more or less distinctly preserved under all circumstances. No other trilobite in the 
lower strata possesses this character; the Ceraurus pleurexanthemus having a double row 
of less prominent ones, while in the C. vigilans every second or third segment only is 
marked in this manner. 
In order to illustrate this species, and, if possible, to clear up the difficulty and obscurity 
attending it, I have given numerous figures of its different parts, and of the same parts 
preserved under different circumstances and in different materials. 
Fig. 2 a. An entire specimen, with the exception of the maxillary shields, which are separated at the 
facial suture, leaving the cephalic shield in the usual form. The crust of the central lobe of 
the thorax is removed, obliterating the tubercles, 
Fig. 2 6. An imperfect specimen, with the maxillary portions partially preserved, but separated at the 
suture and pressed downwards. 
Fig. 2c. The thorax and caudal shield, preserving the crust with the tubercles upon the middle lobe, 
in a very perfect manner. 
Fig. 2 d. The cephalic shield, with the maxillary portions separated at the facial suture. 
Fig. 2 e. The cephalic shield entire, but so much compressed that the eyes are obliterated. The posterior 
angles are too much rounded in the figure; in other respects, the true form of the buckler 
is represented. 
Fig. 2 The cephalic shield preserved in compact calcareous stone. This specimen is from the same 
locality, and very similar to the one on which Eaton founded the Genus Brongniartia. 
Fig. 2 g. A larger specimen, in compact limestone, preserving a more convex form in all its parts. 
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