STRUCTURE AND CLASSIFICATION OF THE FOSSIL CRUSTACEA. 181 



The best example of this is the Belimtrus regina of Baily, from the Irish 

 Coal-measures ; then follows the B. trilobitoides, of Bnckland, the B. an- 

 thrax, Prestwich, the B. arcuatus, Baily, and lastly, the B. rotundatus of 

 Prestwich. By placing these forms in the order indicated, we find a gradual 

 change from the less to the more anchylosed condition of the body-segments, 

 which attains its greatest concentration in the recent Limulus. 



But besides these, we have in Hemiaspis a form more separated into distinct 

 segments than is Belinurus regina ; so that the passage from. Eurypterus to 

 Belinurus, and from that again to Limulus proper, seems capable of being 

 bridged over, and we are justified in placing them in the same order, though 

 separated into distinct subdivisions. 



We have adopted Dr. Dana's name of Meeostomata for the order, making 

 the first suborder, EtriiTPTEBinA, to contain : — 



A. 



1. JPterygotus, Agassiz, having 14 species. 



51 species. 

 Second suborder, Xiphosttba, to contain : — 



A. 



1. Belinurus, Konig, having 5 species. 



B. 



2. Limidus, Miiller „ 14 „ 



19 species. 



I characterize the order Meeostomata as Crustacea having the mouth fur- 

 nished with mandibles and maxillae, the ai^pendages to which fulfil the func- 

 tions of limbs, becoming walking- or swimming-feet, and organs of prehension. 



Suborder Euetpteeida, Huxley, 1859. — Crustacea with numerous free 

 thoracico-abdominal segments, the first and second of which bear one or more 

 broad lamellar appendages upon their ventral surface, the remaining segments 

 being devoid of appendages ; the anterior rings united into a carapace bearing 

 a pair of larval eyes near the centre, and a pair of large marginal or subcen- 

 tral eyes ; the mouth furnished with a broad postoral plate or metastoma, and 

 five pairs of moveable appendages, the posterior of which form great swim- 

 ming-feet, — the telson or terminal joint being extremely variable in form, and 

 the integument characteristically sculptured. 



XiPHOsuEA (Gronov.). — Crustacea having the anterior segments welded 

 together to form a broad convex buckler, upon the dorsal surface of which are 

 placed two larval frontal eye-spots, and two large lateral compound eyes. 

 Beneath this shield-like covering is j)laced the mouth, furnished -with a 

 small labrum and a rudimentarj' metastoma, and six pairs of moveable appen- 

 dages. Posterior segments of the body more or less free in the fossil 

 species, but anchylosed together in the recent species, and bearing upon 

 their ventral surfaces a series of broad lamellar appendages. The telson or 

 terminal segment ensiform. 



