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



head-shield shows many points of resemblance; the p3'gidium is 

 similar, and the short furrow on the anterior edge of the inner 

 portion of each pleura of the thorax may possibly correspond to that 

 in AmpMon, which Salter says exists beneath the crust. There are 

 only twelve body-segments in the single species which has been 

 recorded. From want of material for examination I cannot say 

 more about its affinities. 



Tiae remaining genera can have their relations indicated with more 

 certainty. The genus Spharexochns, as described in my previous 

 article, is led up to by Pseudosphcerexoclius. In the latter the posterior 

 branch of the facial suture cuts the outer margin of the head-shield 

 only a short way in front of the genal angle, and the basal lobe of 

 the glabella is incompletely circumscribed ; in Sphterexochus the 

 suture cuts the margin at the genal angle itself, and the basal lobe 

 is completely circumscribed by a strong furrow. In Pseudosphcer- 

 exoclius the body-segments are twelve in number and the rounded 

 pleuree bear a nearly obsolete line of puncta ; in Sphcerexochus the 

 body-segments are reduced to ten,^ and the last traces of the puncta 

 have completely disappeared ; the pleuree also are shortened by the 

 reduction of the length of their outer portion. In Pseudospliairexoclius 

 the pygidium shows four pairs of pleuras with long pointed free 

 terminations ; in Sphcerexochus the number is reduced to three — or 

 even to two in one species,^ Sj^h. latens — the last pair or pairs having 

 disappeared, perhaps by fusion followed by absorption, as suggested 

 by Sph. latens and by Sph. bohemicus,^ where the second pair forks. 



It is especially interesting to find in this highly specialized 

 trilobite — the culmination of one line of diiferentiation of the 

 Cheirurid stock— that there is a tendency to revert to the more 

 primitive condition of the Opisthoparia by the migration backwards 

 towards the hind border of the head-shield of the marginal point 

 of section of the posterior branch of the facial suture. The free 

 cheeks are connected by a narrow linear band representing the 

 epistome which is apparently fused with them, for the facial sutures 

 unite in front of the glabella and no twin sutures connecting them 

 with the hypostomal suture have so far been discovered. It would 

 seem as if the obliteration of these connecting sutures was another 

 fact in evidence of the high specialization of the genus. I do not 

 attach excessive importance to the fact that the cheeks are only 

 ornamented with fine tuberculations, and lack the characteristic pits 

 of the typical Cheiruridfe, for we know how completely well-defined 

 and important surface markings, such as pleural and glabellar 

 furrows, may become faint or disappear, how the pleural puncta of 

 some forms may gradually be obliterated, and how tubercles may 

 vary in size and be present or absent in closely allied species. 

 Barrande has figured and described the hypostome of Sphcerexochus, 



1 Salter (Mon. Brit. Trilob., p. 78) says there are eleven segments, but only 

 figures ten. 



- Barrande, Syst. Sil. Boh., vol. i, Suppl., p. 113, pi. ix, fig. 3. 

 3 Ibid., p. 112, pi. vii, figs. 5, 6. 



