55o 



CRUSTACEA. 



followed, probably, by three free abdominal rings, of which only two have 

 been preserved. A long spiniform telson may be conjecturally added to 

 complete this ancient Limuloid Crustacean. In the Devonian rocks, the 

 only genus of Xiphosurans which has hitherto been recognised with cer- 

 tainty is the Protolimulus of Packard, which appears to be nearly related 

 to Neolimulus. In the Carboniferous rocks, on the other hand, the 

 Limuloids are represented by a number of forms, the most important 

 genus being Prestwichia (fig. 414), which has the general form of Neo- 

 limulus, but in which the thoracic and abdominal segments are not 

 marked off from one another, and are all anchylosed. The genus Eup- 

 roo'ps, from the Coal-measures of North America, is hardly separable 

 from the preceding, but the eyes are situated on the anterior edge of the 



Fig. 413. — Neolimulus falcatus, enlarged 

 about three times. Silurian. (After Henry 

 Woodward.) 



Fig. 414. — Prcstivichia rotitndata. 

 Coal-measures. 



cephalic buckler. Another well-known Carboniferous genus is Beliuurus, 

 which agrees with the preceding in having five thoracic and three abdo- 

 minal segments, together with a long tail-spine, but in which the thoracic 

 rings are free and movable, while those of the abdomen are anchylosed 

 with one another. It would also seem probable that the singular Crus- 

 tacean fossils which have been described from the Coal-measures under 

 the name of Cyclus, are really, as maintained by Dr Henry Woodward, 

 the young forms of such Carboniferous Xiphosurans as Belinurus and 

 Prestwichia. The very similar Triassic fossils, for which the generic 

 name of Halycine has been proposed, are probably also the larval forms 

 of Limuloid Crustaceans. 



The family of the Limulida is distinguished from that of the 

 He7niaspid.ee by the fact that the cephalic and thoracic segments are 

 amalgamated to form a cephalothoracic shield, the upper surface of 

 which bears two compound eyes and a pair of simple eyes. The 

 abdominal segments are amalgamated with one another to form a 



