Monograph of the Crustacea of the Chicimmtl Group., 125 



middle and most convex portion termed the "axis," and the Literal, 

 lobes or pleurae ; each segment has the axis divided,, in many genera, 

 into a raised, posterior, exposed portion, and a depressed anterior arti- 

 cular portion, which is concealed under the preceding segment when 

 the animal is extended, becoming visible as it rolls up, or on the frac- 

 ture of the edge of the preceding ring ; the pleiirce are straight or bent 

 down at the end ; in the latter case the end often " sharpened at its 

 anterior edge for contraction," or provided with a triangular "/acei,'' 

 one of its sides parallel with the anterior edge, its base at the distal ex- 

 tremity, and its apex reaching about one third toward the axis, its 

 thickened posterior margin joining the anterior margin of the segment 

 at an obtuse angle, at the point where the end of the pleura begin s to 

 bend down, has, from its form, been called the "knee" by Pander 

 Portlock, etc. ; the same point has been by others termed the "ful. 

 crum.'' In most cases a deep groove extends transversely from the 

 axis along the middle of each pleura as far as the " knee," and then 

 its extremity is often bent backward at an obtuse angle. I propose to 

 term this the "pleural groove," and Dr. Burraeister suggests, with 

 great probability, that the membraneous gill feet were attached to the 

 corresponding ridge on the under side ; caudal shield, or " pxjgidium,^'' 

 semi-elliptical, of one solid piece, axis and side lobes generally with 

 transverse false segmental furrows, rarely corresponding in number 

 on the axis and sides, and not admitting of independent motion. 



Burmeister says, " That the Trilobites do not belong to any of the 

 still living families of the Crustacea, ]3Ut represent a distinct group, 

 most nearly related to the Aspidodraca ; that their organization, how- 

 ever, exhibits pecularities, which at the present day do not occur 

 together in one family, but are dispersed in several heterogeneous 

 groups — while trilobites corresjDond in many essential points, if organi. 

 zation with the Aspidostraca, and are not related to any of the still liv- 

 ing groups of Crustacea, there are various important and even typical 

 differences between them. These differences consist principally in the 

 numerical proportions of- the thoracic rings, since, although the latter 

 certainly vary among the Aspidostraca, they may yet be reduced to 

 several constant fundamental numbers (6, 9, and 12) ; whilst the trilo. 

 bites only exhibit a constant number of rings for each separate genus, 

 and the total number can not be reduced to certain, unchangeable, 

 fundamental numbers, or numerical types. (In attempting to ascer- 

 tain with certainity the number of thoracic rings, we certainly meet 

 with the obstacles that we do not know, nor ever can know, the posi- 

 tion of the sexual openings among the trilobites, which position alone 

 indicates with certainty the boundary of the thorax, etc.) * ^ * 



