440 Proceedings of the Royal Physical Society. 



extremely narrow isthmus of sculptured test.^ The lateral 

 or compound eyes rise out of the lateral grooves, one on 

 each side, a little in advance of the ocelli. They are placed 

 on elevations, which rise more steeply along their outer 

 than on their inner side. The corneal portion of each eye is 

 reniform, with the convex side outwards, and arranged so as 

 to look forwards, outwards, and upwards. It is bounded by 

 folds of test which resemble eyelids. The cornea is divided 

 into numerous minute facets, arranged in oblique lines, 

 ranging forwards and downwards from the upper to the 

 lower eyelid. Along each line the facets number from 12 to 

 14 to the mm., according to the amount of distortion the 

 specimen has undergone during fossilization. 



The test is folded downwards and inwards for a short 

 distance along the posterior margin. Along the three other 

 sides it is folded in to a much greater extent, and can be 

 seen to reach inwards to almost beneath the compound eyes. 

 Folds of this flange are seen to commence near the antero- 

 lateral margins on each side, and to pass forward and 

 slightly diverge from the marginal lines. They evidently 

 meet in the middle line, though the dorsal portion of the 

 test preserved in the fossil obscures that part of the flange. 

 This doubtless corresponds to the fold which simulates the 

 horse-shoe on the under side of the carapace of Limulus, 

 and from which it has derived its familiar name of horse- 

 shoe crab. 



The carapace both on the dorsal and ventral side, except 

 upon the united portion of the eyes and along the posterior 

 margin, is everywhere else sculptured with the characteristic 

 squamose ornamentation, which varies in pattern with the 

 region on which it occurs. As a rule, it is coarser and 

 bolder upon the elevations, and finer and more minute upon 

 the depressed areas. A fringe of larger squames borders the 

 carapace near the posterior margin. The tumid portions, 

 which lie between the median and the side grooves, are very 

 coarsely sculptured, while in the depressions themselves 

 the squames are so minute as to require a strong lens to 

 show them. The elevation on which the ocelli are placed 



1 The oval shape of the ocelli may be clue to distortiou from compression. 



