14 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 73 



the lower parts of the rocky mainland riclge. At such times many 

 limbs and trees on these keys are torn off or uprooted and with their 

 load of Liguihs may be easily carried across and landed on the higher 

 mainland. There it is easy for the snails to crawl off and get into 

 the hammocks, where they are soon established. 



The last general earth movement in lower Florida was a slight sub- 

 sidence which may be continuing yet in places and of which there is 

 abundant and widespread evidence. In the Shark River region 

 hundreds of acres of what must have been land are to-day under a 

 very shallow sheet of water in which littoral forest grows thickly and 

 in the same general area and in Biscayne Bay are dead trunks 

 of large mangroves that probably started growth on the land, 

 but have been killed by the slight subsidence. The marly ham- 

 mocks along the south coast are tumbling into the sea and the general 

 chain of keys is being rapidly destroyed by the encroaching ocean. 

 The great land bridge has been reduced to low islets and shoals. 

 This subsistence has had a marked effect on the Liguus of the 

 upper islands. 



The distribution of the Liguus on the Upper Keys is amazing, and 

 for a long time I was utterly unable to understand how it came about. 

 Seven subspecies — castaneozonatus^ roseatus^ Jineolatus, vaca^n^sis, 

 marTnoratus^ luteus, and subcrenatv^ — are found living on the upper 

 and lower ends of the chain but all save lineolMus and subci^e^iatus 

 are entirely absent from the central part of it. The subspecies sub- 

 crenatus now inhabits Lower Matecumbe and has probably lived in 

 the islands to the southwest as far as Grassy, perhaps to Vaca, while 

 castaneozonatus and liiteus occupy the northern end of Upper Mate- 

 cumbe and the islands to the northwest. Long Key, farther down in 

 the chain though a large island with some dry, hammock-covered 

 land, seems to be absolutely lacking in Liguus. Yet on Lower 

 Matecumbe, Lignumvitae, the extreme lower end of Upper Mate- 

 cumbe and Indian Key, lying in the exact region where five of the 

 forms I have mentioned are absent, we find no less than four tolerably 

 well-defined subspecies of solidus, a spe.cies only found elsewhere in 

 the United States on the Lower Keys. Why are these seven forms 

 present in the ends of the chain and all but two absent from the 

 center, and why should they be replaced here by a species of the lower 

 islands? Why is Long Key without Liguus when it seems to be 

 perfectly adapted for their growth ? 



I feel sure that during the period of greatest elevation the entire set 

 of the Upper Keys from near Cape Florida to and including the Vaca 

 group was one great island, so high and well clothed with hammock 

 that the seven forms I have mentioned, which were early arrivals, 

 became distributed throughout the whole. Then at the time of 

 the last general subsidence the middle of this chain went down just 



