184 BRITISH FOSSIL CRUSTACEA. 



basal joints are always subservient to alimentation ; whilst in Stylonurus two of the most 

 posterior pairs of appendages fulfil the function of natation, being developed into long 

 slender oars, which upon occasion might also have served as claspers by which to anchor 

 itself to any floating body when resting or in quest of food. 



Immediately behind the mouth and attached to the hinder portion of the head, with 

 its free border extending forward so as to overlap the bases of the maxillipeds and the 

 oral aperture, is the great metastoma or post-oral plate, which is present in all the 



EURYPTERIDA. 



The compound head is succeeded by twelve free and movable segments, of which, 

 only the first two (?) bear appendages, the succeeding somites being apparently destitute 

 of any organs. These post-cephalic appendages consist of two or three pairs of lamelli- 

 form plates (the modified thoracic 1 limbs) united together down the mesial line and 

 attached to the ventral surface of the somites, against which they fit when closed, thus 

 serving as defensive coverings or sheaths to the organs of generation and respiration, 

 which they bear upon their inner and upper surface (see ante, Part III, pp. 114 — 119, 

 figs. 35—38). 



The twelve free segments are terminated by a post-anal plate (or " telson ") broadly 

 lanceolate or bilobed in Pteryyotus (Parts I and II) and Slimonia (Part III), and 

 ensiform in Eurypterus and Stylonurus (Part IV). 



Viewed as a whole, the Eurypterida present, to my mind, the clearest evidence of a 

 group essentially fitted for easy and tolerably rapid motion, their long and gradually 

 tapering body undoubtedly enclosing within it a correspondingly compact and solid mass 

 of muscles, by which to move it at will through the water, the broad hastate telson (in 

 Pteryyotus and Slimonia) serving both as a powerful propeller and also to depress or elevate 

 its course, whilst its great spatulate swimming- feet would increase its speed or change 

 its lateral direction ; lastly, the chelate antennae seem admirably fitted to seize its prey 

 either when overtaken or approached by stratagem. 2 



All the genera belonging to this remarkable Palaeozoic group, the Eurypterida, are 

 now extinct. (See ante, Parts I — IV.) 



II. — In the second division of the Merostomata, about to be described, the Xipho- 

 sura, of which the modern Limidus may serve as the type (see Part I, PI. IX, fig. 1, and 

 PI. XXXIV — XXXVI), the head is composed of a broad, semilunar shield, the dorsal 

 surface of which rises in the centre, forming a median ridge, upon the fore part of which, 



1 Thoracetron, Owen, op. cit. 



2 It is highly probable that the chelate antennae in Pterygotus may have also fulfilled the function 

 of claspers in the males, being perhaps longer and stronger than in the female (as we find is commonly the 

 case with the chelate (3rd thoracic) pair in Decapod Crustacea) ; or they may have been simple palpi in 

 the female, in which case the genera Slimonia and Eurypterus with simple antennae would have to be 

 considered as females, and Pteryyotus with its chelate antennae as a male, the opposite sex in each case 

 being unknown, or undetermined. 



