208 F. R. Coicper Reed — On the Cheiruridce. 



place, the absence of facial sutures separates it from all tbe other 

 genera in the Cheirurid^, while this feature, combined with the 

 absence of eyes, the pentamerous lobation of the head, the expanded 

 termination of the glabella, and the presence of the pleural row of 

 puncta on the neck segment (in A. Boliemica), and the close 

 resemblance of this segment to the thoracic segments are larval 

 features which indicate a very low stage of development and 

 a comparatively small amount of differentiation. The Bohemian 

 species of the subgenus JEccoptocheile resemble it in the row of 

 puncta on the inner portion of the pleura, the number of the 

 thoracic pleurae, and the notch on each side of the glabella in 

 the front border of the cephalon ; and Barrande himself remarks 

 that Areia approaches most closely the species Ch. claviger, 

 globosus, etc., belonging to the s.g. Eccoptoclieile. The hypostome is 

 analogous to that of Cheirurus, and the ornamentation of the cheeks 

 is similar. As I have remarked in my previous article on 

 Cheirurus, the pygidium appears frequently to follow an inde- 

 pendent line and different rate of development to the other parts 

 of the body, and is, therefore, of doubtful value in tracing affinities. 

 In Areia it shows only two segments on the axis and two pairs of 

 pleurae. It is only in NieszTcowsJcia and in one species of SpTicerexocJius 

 (Sph. latens, Barr.) amongst the Cheiruridse that we find the pleui-ge 

 of the pygidium numbering two. But this does not show affinity, 

 for in NieszkowsMa there has been reduction, absorption, and. 

 crowding out of the last pair by the hypertrophy of the first; in 

 Sph. latens there has been fusion of the last pair of pleuree into 

 a terminal piece; but in Areia the second and third pairs of pleurse 

 alone seem to have been developed. 



The fact that the Cheirurid^ show on the whole a high degree of 

 differentiation and specialization, and. have in the main lost the 

 features of immaturity, while on the other hand the Encrinurid^ 

 show a much lower stage of development and retain more larval or 

 primitive characters, leads Beecher to place the genus Areia in 

 the latter family. But from the several points of resemblance of 

 this genus to the early species or subgenera of Cheirurus, and 

 from its retention to maturity of certain larval and early phylo- 

 genetic features exhibited in some genera or subgenera which 

 undoubtedly belong to the Cheiruridce, I am inclined to place it in 

 this family, and to regard it as a primitive form in which the 

 ordinary ontogenetic development has been irregularly arrested. 



Turning now to Placoparia, we find also in it many primitive 

 features. The dorsal furrows are usually described as bifurcating in 

 front, one branch running forwards and the other turning outwards 

 at right angles. This latter branch may possibly represent one of 

 the furrows we find on the cheeks in Areia. At any rate, the narrow 

 marginal cheeks, combined with the absence of eyes, are primitive 

 characters found only in the larv^ of higher trilobites. The facial 

 suture ends also at the genal angle, recalling the less hMily 

 differentiated Opisthoparia of Beecher, in which the facial smure 

 cuts the posterior border of the head- shield. The pentamerous 



