41 



Crustacean (incipient, it may be), what are its nearest allies in that class ? Do the 

 grounds on which I reject a ' Trilobiten-Stadium ' at any period of its larval life meet 

 with any support from affinities manifested by the adult to other Crustaceous forms ? 



Pterygotus and Eurypterus resemble I/imulus in the organs of vision, save that the 

 facets of the large lateral compound eyes are less distinct or less conspicuous in the fossil, 

 possibly exuvial, specimens of those extinct forms. 



Both palaeozoic genera manifest a clear and exclusive affinity to Limulus in the general 

 proportions, modifications, and functions of the cephaletral limbs. In Fig. 15. 



Fterygotus (fig. 14) the foremost pair is chelate, the hindmost pair 

 lamellate, the intermediate pair are less differentiated and are alike. 

 In Slimonia (fig. 15) the foremost pair is the smallest and shortest, 

 the hindmost tlie longest, and it is also lamellate. In both genera all 

 the cephaletral limbs, at least all but the foremost, had the basal joints 

 beset with ' carding-spines,' showing their functional subserviency, as 

 in Limtilus, to the mouth as preparatory organs of digestion. 



We may consequently infer, from the analogy of the food of the 

 living King-crabs, that Nereids and other soft-bodied Annelids abounded 

 in the sandy or muddy beds of the old ocean in which the Merosto- 

 mata * burrowed. 



In these the cephaletral shield (figs. 13, 14, a) was small, both in 

 breadth and length, as compared with that in Limulus, Frestwichia 

 (fig. 17), and Bellinunis ; but it was similarly shaped as regards the 

 curved anterior trenchant fossorial margin. The mouth was inferior, 

 bounded laterally by the carding-joints, and posteriorly by a ' labium,' or connate chi- 

 laria, of large size — and, if homologous with the parts in like relation to the mouth of 

 Limulus (*, *, Plates II., II. A, III., & IV.), dififering therefrom by the pair continuing the 

 condition shown by the thoracetral limbs of Limulus. Whatever homology be adopted, 

 the hindmost of the ' trophi,' or oral organs, is single and symmetrical in Eurypterids. 

 No fossil Merostome has yet been discovered showing more than three pairs of cephal- 

 etral jaw-limbs between the foremost and hindmost pairs. Thus there is one pair less 

 than in Xiphosures (fig. 8). In the fine fossil exuvium of the young Fterygotus anglicus 

 figured by H. Woodward in his excellent ' Monograph on the Merostomata ' (plate ii. 

 fig. l)t, there seem to be as many as five Hmbs on the left side, with spinigerous 

 haunches ; but it is uncertain whether the foremost of these may not be the fellow of the 

 second, displaced from the right side. This, therefore, leaves the forcipate antennae, or 

 foremost pair of jointed cephaletral limbs, devoid, like that pair in Limulus, of the basal 

 carding jaw-plate. 



I think it of less moment to speculate as to which of the six pairs (ii-vii) of cephal- 

 etral limbs in Xiphosures were undeveloped in Eurypterids, than to realize the certain 

 correspondence of character of the five developed pairs in the latter family with those 



* This term signifies, and most aptly in its present adopted extent of application, the peculiar structure and func- 

 tion of the cephaletral limbs described in previous paragraphs. 

 t Vol. of the Palaeontographical Society for 1866. 



Slimonia acumi- 

 nata, Wd. 



