36 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 21, 



which takes place in the respiratory organs of certain Amphibia, 

 in which the branchiae become replaced by a lung. 

 The following is a diagnosis of the 



Order MEROSTOMATA, Dana, 1852. 



Having the mouth furnished with mandibles and maxillae, the ter- 

 minations of which become walking- or swimming-feet and organs 

 of prehension. 



I. Suborder Eueypterida, Huxley, 1859. 



Crustacea with numerous free thoracico-abdominal segments, the 

 first and second (?) of which bear one or more broad lamellar appen- 

 dages upon their ventral surface, the remaining segments being 

 devoid of appendages ; the anterior rings united into a carapace 

 bearing a pair of larval eyes (ocelli) near the centre, and a pair of 

 large marginal or subcentral eyes; the mouth furnished with a 

 broad postoral plate, or metastoma, and five pairs of moveable ap- 

 pendages, the posterior of which form great swimming-feet ; the 

 telson, or terminal segment, extremely variable in form ; the integu- 

 ment characteristically sculptured. 



1. P teryffotus, Agassiz 16 species. 



2. Slimonia (Page), H. Woodw 3 „ 



3. iSz^yfowwrws (Page), H, Woodw 6 „ 



4. Eurypterus, De Kay 22 „ 



Subgenus BoUchopterus, Hall 1 , , 



5. Adelophthalmus, Jordan 1 ,, 



6. Bunodes, Eichw 2 „ 



7. Arthropleura, Jordan 3 „ 



8. Hemiaspis, H. Woodw 6 „ 



9. Exapinurus, Nieszk 1 ,, 



10. Pseudoniscus, 'Nieszk 1 „ 



62 



II. Suborder Xiphosuea, Gronovan, 1764. 



Crustacea having the anterior segments welded together to form a 

 broad convex buckler, upon the dorsal surface of which the com- 

 pound eyes and ocelli are placed, the former subcentraUy, the latter 

 in the centre in front ; the mouth furnished with a small labrum, a 

 rudimentary metastoma, and six pairs of moveable appendages. 

 Posterior segments of the body more or less free, and bearing upon 

 their ventral surfaces a series of broad lamellar appendages ; the 

 telson or terminal segment ensiform. 



1. Belinurus {X.dnig),'Ba,ily 4 species. 



2. Prestwichia, gen. noy 2 „ 



3. Limulus, Miiller 15 „ 



21 



Of the Eurypterida, 49 species occur in the Upper Silurian, 18 in 

 the Devonian, and 10 in the Carboniferous formation. Of the Xi- 

 phosura, 6 occur in the Carboniferous, 1 in the Permian, 1 in the 



