122 G. F. MATTHEW: ILLUSTRATIONS OF 



parent. Except the missing types, 6 and "7, all the Solva trilobites are represented by- 

 corresponding forms in the Acadian fauna, which should therefore be compared to the 

 Solva or Longmynd fauna rather than to the Menevian : — 



Solva Group. 



1. Microdiscus sculptus. Hicks. 



2. Conocoryphe Lyelli, " 



3. " Solvensis, " 



4. " bufo, 



5. Paradoxides Harknessi, " ' 



6. " aurora, Salter. 



7. Plutonia Sedgwickii, Hicks. 



8. Agnostus Cambaensis, " 



Acadian Fauna. 

 Microdiscus Dawsoni, Hartt. 

 Ptycopliaria Robbii, &c., Harlt (sp.) 

 Ctenoceplialus Mattliewi, " " 

 Conocoryphe elegans, " '' 



Paradoxides Eteminicus, Mattlutv. 



(?) 



(?) 

 Agnostus (sp.) undescribed. 



Another point bearing upon this question is the development of the eyelobe in Para- 

 doxides. In my first article read before this Society, on the Paradoxides of the St. John 

 Group, it was shown that, among the changes of form in various parts of the cephalic 

 shield which occvirred during the growth of the individual trilobite, the shortening of the 

 eyelobes was a distinct feature. It is true that even in the adult stage, all the Acadian 

 Paradoxides haA^e continuous, or nearly continuous, eyelobes, and therefore the contrac- 

 tion of this member is not conspicuous ; but there being such a change, even to a small 

 extent, during the growth of the animal, a continuous eyelobe is likely to be an embryonic 

 feature of the later Paradoxides, and suggests the inquiry as to whether there was a 

 corresponding change in the sj^ecies of Paradoxides as they occur in chronological 

 succession. 



Taking the species of the British Cambrian rocks as a criterion, there may be 

 observed in the Middle Solva beds the species P. Harknessi with continuotis eyelobes. 

 Advancing a grade higher, there is found in the Upper Solva beds the species P. aurora, 

 with half-short eyelobes, a rather small species. Next there is found in the Lower Mine- 

 vian the species P. Hicksii, in which the contraction of the eyelobes has proceeded so far 

 as to leave a suture behind the eyelobe as long as the eyelobe itself But the greatest 

 advance of development in respect of the eyelobe is manifested by the great Paradoxides 

 of the Welsh measures, P. Davidis, whose oval eyelobe has left behind it a suture twice 

 its length. This species belongs to the Middle Minevian. The shortening of the eye- 

 lobe in the "Welsh Paradoxides therefore corresponds to the geological age of the species. 

 As the Middle Solva beds are the British strata which hold the Paradoxidean forms 

 equivalent to the species of the Acadian measure, the latter may be regarded as older 

 than Menevian. 



Another point, which is worthy of consideration in this connection, is the peculiar 

 dorsal siiture of the Acadian species of Conocoryphe. This does not agree with the suture 

 of the species taken as the type of this genu.s, nor with that of any in the Menevian group 

 proper, but if it be compared with the suture of C. bufo of the Solva group a very close 

 resemblance is apparent. But while this suture is shown for one only of the Welsh Cam- 

 brian species, it belongs to three of the species of the Acadian fauna. 



As the Conocoryphe of the St. John group differ in the course of the suture from the 



' Dr. Hicks compares this species with P. rugidosus (Corda), which is very closely allied to the Acadian si^ecies 

 P. Eteminicm. 



