UTICA SLATE AND HUDSON-RIVeR GROUP. 251 



The buckler of our species bears considerable resemblance to that of specimens of Olenus 

 gibhosus from Sweden ; but the true Olenus., as in Paradoocides, 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 Oleni. 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 Emmrich 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 

 Calym,ene 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 diflferent 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 b. An imperfect specimen, with the maxillary portions partially preserved, but separated at the 



suture and pressed downwards. 

 Fig. 2 e. 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 «. 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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