THE EURYPTERIDA AND XIPHOSURA COMPARED. 231 



not clearly indicated in any annulose animal, the presence of a pair of appendages always 

 marks the existence of a separate segment. 



It is not beyond possibility that a second pair of antennae may yet be discovered in 

 the genus Slimonia, in which their presence has been suspected ; but in such a case as 

 that of the supposed suppression of one pair of these organs there is warranty for the 

 theory to be derived from other living Crustacea 1 besides that afforded by a comparison 

 with Limulus. 



Neither the space allotted me, nor the time at my command, permit me to reply at 

 greater length to Dr. Packard's valuable remarks. To answer them as fully as I could 

 wish would involve an additional part to this Monograph. 



I have endeavoured to the best of my ability to interpret aright the fossil remains of 

 the ancient order of the Merostomata, and I must leave the onus probandi to those who 

 object to that interpretation. 



Whilst tendering my sincere thanks to Professor Owen for the valuable assistance I 

 have derived from his memoir on the American King-crab, I would venture to touch on 

 one or two points in which I beg leave to dissent from his conclusions. 



1st. " No observer," says Professor Owen, " has yet testified to their capacity of 

 uplifting the body from the ground, whether dry or submerged, and of bearing it along by 

 successive steps, as do the jointed legs of the Isopod, the Lobster, or the Crab" (p. 495). 



To this I would reply that many years since I carefully observed the living Limuli in 

 the Fish-house at the Zoological Gardens, Regents Park, and I have there seen Limulus 

 trot briskly along upon the floor of the tank, keeping the edge of its cephalic shield 

 raised clear above the level of the sand. 



In the spawning season the female laden with eggs not only walks to shore, but 

 carries on her back the salacious male holding on firmly to her carapace by his specially 

 modified antenna. " When left exposed by the tide they hasten (says Dr. Lockwood) to 

 overtake it." 



2nd. "In Pterygotus, the antennas," writes Professor Owen, " are forcipated members 

 for the prehension of food, as in Limulus. The three succeeding pairs of limbs are still 

 less capacitated, through their smaller size and slenderness, for gradatorial movement of 

 the body. They are adapted to rout out of the sand or mud disturbed by the spade- 

 shaped head the objects of food which the front pair is modified to seize. The larger 

 terminal pair of limbs are more decidedly natatory in form than the corresponding 

 lamelligerous pair in Limulus." 



We infer from the spatulate form of the great pair of swimming-feet and the rounded 

 and elongated body that Pterygotus was a fairly active swimmer, and that it did not 

 burrow. In fact, the position of the compound eyes upon the edge of the head precludes 

 the idea of its front border being used as a shovel like that of Limulus. 



1 The females of Brachyscehts are destitute of the inferior pair of antennae ; the male possesses them 

 like other Amphipoda. Spence Bate and Fritz M tiller find no trace of them in the young. 



