CRUSTACEA. 17 1 



Body-rings eleven in number. Pygidium with an undefined 

 axis and broad limb. The family includes the single genus 

 Lichas, and extends from the Lower Silurian into the De- 

 vonian. 



9. PROETID^E. Cephalic shield with the facial sutures not 

 uniting in front of the glabella. Body-rings nine or ten in 

 number. Axis elongated. This family ranges from the Lower 

 Silurian to the Carboniferous. Besides Proetus, this family in- 

 cludes Phillipsia and Griffithides. 



10. ACIDASPIM;. Surface ornamented. Body-rings from 

 eight to ten in number. The ends of the pleurae extended into 

 spines, and directed backwards. Pygidium of from two to 

 three segments, small, furnished with prominent spines. The 

 genus Acidaspis ranges from the Lower Silurian into the De- 

 vonian. 



11. BRONTEIM:. Body-rings ten in number. Pygidium 

 large, the axis extremely short, the margin entire. The family 

 includes only the genus Bronteus (fig. 105), and is confined to 

 the Upper Silurian and Devonian Rocks. 



12. HARPEDID.E. Cephalic shield horse-shoe-shaped, its 

 lateral angles greatly prolonged. Facial suture cutting the 

 exterior margin of the buckler. Body-rings numerous, some- 

 times no less than twenty-six in number. Pygidium small. 

 This family comprises only the genus Harpes, all the species 

 of which belong to the Lower Silurian, Upper Silurian, and 

 Devonian. 



13. CYPHASPID^E. Cephalic shield with the posterior angles 

 usually prolonged into spines, the facial suture cutting its ex- 

 terior margins. Body-rings from ten to twenty-two. Crust 

 spinulose or pitted. The chief genera of this family are 

 Cyphaspis and Aulacopkura (Arethusina], the former ranging 

 from the Lower Silurian to the Devonian, the latter exclusively 

 confined to the Silurian series. 



ORDER MEROSTOMATA. 



Crustaceans, often of large size, in which the mouth is furnished 

 with mandibles and maxillce, the terminations of which become 

 walking or swimming feet, or organs of prehension (figs. 117, 

 1 1 8). 



The order Merostomata comprises the two sub-orders of the 

 Xiphosura and Eurypterida. The former appears to have 

 commenced its existence in the Upper Silurian period, and is 

 represented at the present day by the Limuli or King-crabs. 

 The latter is wholly extinct, and is exclusively Palseozoic, none 



