534 Mr. T. H. Withers on a 



adult female, these pleopods must afterwards re-develop, thus 

 ))roviding an example of a phenomenon somewhat uncommon 

 in the animal kingdom, the degeneration of a well-developed 

 and functional organ, and its subsequent re-development at a 

 later stage. 



Two specimens of G. lalandii which are in the possession 

 of the Natural History Museum, South Kensington, and 

 which were collected by Dr. C. W. Andrews at a spot about 

 200 feet from the summit of Christmas Island, are respec- 

 tively a male measuring 18"5 mm. across the carapace and a 

 female measuring 18 mm. 



At this stage the abdominal appendages have already 

 assumed the characters of the adults, although the size of 

 the chelse and abdomen are still the same in both sexes. 



Sex-differentiation of the pleopods, therefore, must take 

 place between the first young stage measuring 4"2 mm. across 

 the carapace and the attainment of a size of 18 mm. ; and 

 during this period the young crab leaves the sea and migrates 

 inland to take up its future abode in the wooded districts of 

 the interior. 



I am indebted to Dr. G. W. Andrews and Dr. W. T. 

 Caiman for the loan of specimens and for helpful informa- 

 tion and advice. 



LIV. — On a neio Brdchyurous Crustacean from the Upper 

 Cretaceous of Jamaica. By Thomas H. Withers, F.G.S. 



(Published by permission of tlie Triistees of the British Museum.) 



[Plates XVI. & XYII.] 



Dr. D. Woolacott recently presented to the Geological 

 Department of the British Museum some crab-remains 

 collected by him from the Upper Cretaceous of Jamaica, and 

 these were entrusted to me for description. 



Cretaceous crabs are always interesting, but the interest is 

 greatly increased when, as in this instance, it is possible by 

 careful development to expose many of the appendages, so 

 that the structure can be studied almost as completel}' as in a 

 recent specimen. The form here described is not only one of 

 the most complete Cietaceous crabs so far discovered, but is 



