568 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Castle Harbor, which is an excellent example of one of these large 

 submerged sinks, communicates with the sea through several channels, 

 so that it is comparatively open. The main portion of the harbor is 

 about three miles iu diameter, is nearly round, and has a greatest depth 

 of about seven fathoms ; but the bottom is very uneven. On the south 

 it is separated from the ocean by a chain of small islands, between which 

 there are five narrow channels. On the northwest the enclosing wall is 

 more nearly complete, being broken in only one place, — by the body of 

 water known as Ferry Reach, a partially artificial basin nearly cut off 

 from the harbor by a causeway and an island, but communicating on the 

 west with the open water of the great lagoon, and on the northeast with 

 the passage known as Stocks Harbor. Through this passage Castle 

 Harbor connects directly with St. George's Harbor and througli it with 

 the outer lagoon. Thus Castle Harbor, communicating with the sea on 

 three sides, combines the advantages of a protected location and a free 

 circulation of water, making it a very favorable locality for many marine 

 organisms. The bottom is of clear white sand, interrupted by numerous 

 ledges covered with corals, millepores, and coralline algae. 



A sample of this sand from Station No. 356, near Castle Island, shows 

 the following composition. It is white, slightly flecked with pink and 

 yellow, slightly coherent, sticky ; with 20 per cent of fine fiocculent 

 amorphous calcareous ooze. P^oraminifera are a striking feature, form- 

 ing 40 per cent or more of the mass; particularly abundant are the 

 large sand-dwelling genera Orbitolites, Orbiculina, and Ammodiscus. 

 A partial list comprises Orbitolites duplex, Orbiculina adunca. Ammo- 

 discus tenuis, Cornuspira foliacea, Pulvinulina menardii, and sjiecies 

 of Polystomella, Trochammina, Textularia, Cyclammina, Cassidulina, 

 Biloculina, and a very few Globigerinae. The great majority of them 

 are living, though there are in addition many dead tests and casts. 

 Most important after tlie Foraminifera are the coralline algae Udotea 

 and Halimeda, which Agassiz ('88*, p. 82) has also described as occur- 

 ring in great masses on the Florida Reef. In this particular sample 

 they form only about 10 per cent, but in certain parts of Castle Har- 

 bor they grow in such great abundance that their remains form 30 

 per cent to 40 per cent of the sand. Living mollusks and their shells, 

 mostly fresh and little worn, form 10 to lo per cent, the most im- 

 portant genera being the gasteropods Bulla, Codakia, Nassa, Cardium, 

 Gouldia, Rissoina, Olivella, Modulus, Bittium, Cadulus, and Vermetus, 

 and the bivalves Crasinella, Abra, Macoma, Semele, Area, Lasaea. and 

 Telliua. The remainder consists of serpuline fragments, of millepore, 



