142 G. F. MATTHEW : ILLUSTRATIONS OF 



adïilt form is reached. I may mention one feature observed in some very small head- 

 shields (about 2 mm. long) which disappears in the larger ones. This is the enlargement 

 of the end of the ocular fillet which is next the glabella ; the enlargement is not so great 

 as in the fillet of the Conocorypheaus, but may point to the common origin of the two 

 families, which, as has already been shewn, approximate in other respects in their early 

 stages. 



Among the species described by Prof Hartt is Conocephalites quadratus. But among a 

 large number of head-shields of the smaller trilobites, collected from the same shales from 

 which he obtained this form, I was unable to recognise this minute species. There are 

 no groupings in the layers of the shale of small heads of this type as there are of the 

 species of Microdiscus and Agnostus ; hence it would appear that this form is one of the 

 younger stages of a species already known, probably of L. Ouangondianus. 



It is with considerable reluctance that the writer refrains from identifying this 

 species with Angelin's Lioslraci/s aculeatus. There can be no question of their close rela- 

 tionship ; but the Scandinavian authors later than Angelin, who have described the 

 Cambrian trilobites, are not in accord as to the species indicated by Angeliu under this 

 name. In the preceding remarks, the avithor has named the Scandinavian form which 

 appears to be most closely allied to L. Ouangondianus, but until the parts of the Swedish 

 trilobite are better known, it woiild be premature to accept the name aculeatus for the 

 Acadian species. 



PTYCHOPARIA, Corda. 



The following is the substance of Corda's description of this genus (specific des- 

 cription) : — 



Cephalic shield semilunar, glabella narrow anteriorly ; furrows four, divided ; 

 variations three, and fourth obsolete. Facial sutures widely separated, extending and 

 converging forward from the eyes, so as to intersect the anterior margin within a point 

 where a line would cut it if drawn through each eye parallel with the axis. These lines 

 extend themselves from the eyes to the posterior margin by making a double curve, and 

 cut the margin within or near the lateral angles. 



Thorax, 16 to 19 segments. Pygidium, 3 to 4 rings in the axis. 



Type, P. striata. 



In Corda's figure of P. striata (and also in Barrande's) there are only fourteen thoracic 

 segments and six or seven articulations in the axis of the pygidium. There is thus an 

 error in the above description as to the number of segments in the thorax and pygidium 

 respectively. Corda having apparently counted in the thorax several joints that belong to 

 the pygidium. That the pygidium has the more numerous joints mentioned by Barrande 

 is undoubted, and this is an important distinction between this genus and Soleuopleura. 

 This genus also diii'ers from Soleuopleura in the possession of spines at the genal angles, 

 for while some species of Soleuopleura have prolonged points to the movable cheeks, 

 none have the long spines of P. striata. Such spines are usually associated with points ^(^ 

 the pleural segments, and in this it will be observed that there is also a distinction 

 between P. striata and all the known species of Soleuopleura, except S. Robbii, in which 

 the points are very weak. 



