PLATE XX. 



UPPER SILURIAN CRUSTACKa. 



Order — Merostomata. 

 Sub- Order — Eurypterida. 



Figs. 1 — 4. — Slimonia acuminata, Salter, sp. P. 105. 



Fig. 



1. Upper side. Restored. 



2. Under side. „ 



3. Median appendage of thoracic plate or operculum (male?). 



4. Ditto (female?). 



Segments I — VI. Cephalic. 1 



VII— XIII. Thoracic 2 

 XIV— XX. Abdominal. 3 



' One cephalic segment (that bearing the antennules) is suppressed in the Eurypteridae. 



2 The first thoracic segment bearing the thoracic plate or operculum is coalesced with the head-shield 

 both in Limulus and in Pterygotus, &c. 



3 In the explanation to Plate VIII, the 14th Segment is included as thoracic; this is an error, and 

 should be corrected. 



