160 Ceraurus pleurexanthemus. 



signed to be compared with the several figures in Plate XI, 

 which have been drawn from the combined evidence obtained 

 by the examination of numerous specimens. 



It should be borne in mind, that as the shell, in the fol- 

 lowing description, is supposed to be placed back-downward, 

 as shown in figure B, Plate XI, the words upper and under, 

 etc., when used in this article, are to be taken in their strict 

 sense, as compared with figure B, and not in the sense that 

 they usually have, when a trilobite is placed with its back 

 uppermost. 



Head. Anterior, lateral, and free posterior margins bordered by a 

 " doublure." Glabellar depression, concave, longer than broad, narrowed 

 posteriorly ; anterior margin a semi-lunate curve, to which the hypostoma 

 is attached by a suture (hypostomatic suture) ; from the extremities of 

 this suture, lateral ridges extend to the posterior lateral margins of the 

 central neck depression. Four short processes project obliquely back- 

 ward from each ridge into the glabellar depression. The four anterior 

 processes are rudimentary and concealed by the hypostoma. The four 

 posterior processes have rounded knob-like tubercles upon their upper 

 extremities ; the posterior pair attached opposite the inner posterior 

 angles of the occipital depressions. A low arching ridge separates the 

 glabellar and neck depressions. 



The occipital depressions include the spaces within the "doublure," 

 glabellar ridges, and neck depression. Occipital cavity in the anterior 

 lateral third. Glabellar and occipital depressions finely punctate. 



The neck depression extends laterally as shallow grooves under the 

 " doublure," deepest towards the central depression. Central depression 

 a concave groove, the posterior margin reflected upward and forward, 

 terminating in a thin edge, which articulates with the articular fold of the 

 first thoracic segment. 



The facial sutures arise a little on each side of the centre of the posterior 

 margin of the frontal "doublure, pass forward crossing the "doublure," 

 and curve under its anterior margin, thence obliquely backward to the 

 anterior margin of the occipital cavities, then into those, describing a 

 curve around their anterior lateral bases, and passing out at their pos- 

 terior lateral margins; thence they extend obliquely outward and back- 

 ward to the lateral margins of the cephalic shield at their posterior third, 

 obliquely cut the "doublure," and terminate at its inner margin, at the 

 posterior lateral angles of the occipital depressions. 



Hypostoma subovate, with wing-like extensions of the anterior lateral 

 margins ; central convex portion surrounded by a sinus, and an elevated 

 margin ; this margin, at the anterior half, widens, and forms a slightly ele- 



