DECAPOD CRUSTACEANS OF THE WEST INDIES 21 



All three are troglobitic and almost certainly represent relicts of a 

 once much more widespread epigean stock of which no other trace is 

 known to exist. The ancestral stock must have been a primitive one, 

 retaining exopods on all of the pereiopods, a condition found else- 

 where in the family only in Mesocaris, Paratya, and Xiphocaris. 

 Perhaps the absence of epigean species signifies an inability of the 

 surface members to compete with faunas that later invaded the an- 

 cestral range; however, it is not inconceivable that had the ancestral 

 stock become adapted for a freshwater existence, as have their spelean 

 descendents, one or more Pleistocene inundations of their ranges could 

 have had effects similar to those postulated by Hobbs (1958) in con- 

 sidering the evolutionary history of certain troglobitic crayfishes in 

 Florida. Were there sufficient land remaining in Yucatan, Cuba, and 

 on Mona during such an inundation to serve as recharge areas for the 

 subterranean streams, or if adequate underlying aquifers were avail- 

 able to feed them, those forms that had invaded subsurface waters 

 could have survived, even if the epigean stock were annihilated by 

 salt water. On the basis of the present distribution of the members 

 of Typhlatya, there seems no reason to doubt that the Antillean mem- 

 bers have been derived from stocks that reached the islands from the 

 Central American-Mexican region, probably in Miocene or Pliocene 

 times. 



We are much puzzled by the remarkable occurrence of T. monae on 

 Mona and Barbuda. This disjunct distribution of a troglobite seems 

 almost inconceivable; however, careful comparison of specimens from 

 the two islands reveals no differences worthy of note. That parallel 

 evolution should have resulted in apparently identical populations on 

 the two islands hardly seems possible, but the alternative proposal 

 that a continuous spelean corridor exists or has existed between the 

 islands seems ridiculous. Of course, if it could be shown that the 

 troglobitic facies of T. monae are actually nothing more than eco- 

 phenotypic expressions, then the apparent parallelism is precisely 

 what might be expected in the troglobitic adaptation of an old, 

 stable species. The fact that T. garciai differs from T. monae in com- 

 paratively minor details lends some credence to the latter possible 

 interpretation. 



The monotypic Xijphocaris elongata is a near orphan in the family, 

 retaining the most primitive branchial complement and also having 

 the primitive characters of exopods on all of the pereiopods and un- 

 tufted fingers of the chelipeds. Since it is apparently quite primitive 

 and has no close relative anywhere, one might conclude that it repre- 

 sents the remnant of an old stock that has disappeared elsewhere but 

 in this species has found a congenial habitat in the West Indies. 



