262 THE SPONGES OF THE WEST-CENTRAL PACIFIC 



Ecological Discussion 



The geography and geology of the four archipelagos covered in the 

 present discussion may be briefly described as follows : 



The islands of the Mariana group are all close to the 145th meridian of 

 east longitude and extend from about 13° to 21° north latitude. They are 

 extremely likely to suffer from hurricane or typhoon damage, and they have 

 unpleasant, hot, humid climates. There are abundant evidences of their 

 volcanic origin, but they are much older than the Hawaiian Islands whose 

 volcanos still show plainly. On the other hand, they are younger than the 

 more southerly atolls. Actually much of the land of the Marianas is elevated 

 table reef or reefs. Well-marked terraces indicate several separate uplifts. 

 A good discussion of table reefs may be found in their description by Tayama, 

 1935. He says that they are built up largely by such corals as Pontes, Favia, 

 Acropora, and Leptoria, and (when living) come to have a nearly level sur- 

 face just below the ocean surface. There may be slightly raised rims of coral- 

 line algae about them. 



There are fringing reefs in the Marianas, but only narrow lagoons are 

 to be found. For much of the shore line, the waves of the open ocean break 

 directly on the beach. The lagoons, when present, are usually shallow, and 

 often one may wade to the outer edge. Such a whole structure may be regard- 

 ed as a table reef more recent than the reefs which have been elevated to 

 form the land mass. Tayama's 1939 account refers to them as "apron reefs." 

 Lagoons as wide as one to three kilometers and as deep as 5 meters are found 

 in northwest Saipan (Tanapag Harbor) and extreme southern Guam (Merizo 

 Bay). It was my observation that the bottom of these lagoons was chiefly 

 debris of dead coral, but here and there were little island clumps of living 

 coral. In these lagoons the water temperatures are often very high. 



The islands of the Palau or Peleu group lie at about 134° to 135° east 

 longitude and 7° to 8° north latitude. They have a pleasant tropical climate. 

 Part of their elevated structure is residue of extinct volcanoes, part is elevated 

 marine deposit — now limestone. The largest island, Babeldaub, has an area 

 of 370 square kilometers and may be called a plateau. The limestone is often 

 cemented coral with interstices filled by foraminiferal sand. There are beds of 

 lignite and clay; Pliocene fossils are fairly common. Some fossils as ancient 

 as the Eocene are recorded (Tayama, 1939). It seems possible to correlate 

 some of the terraces with those in the Marianas. 



In addition to the ancient, elevated reefs, there are contemporary living 

 ones, in two series. One line of reefs is very near the shore, seldom more 

 than a kilometer from it, and thus a very shallow lagoon is inclosed. Where 

 the shore is volcanic rock, mangrove thickets are conspicuous. Where the 

 shore is limestone, there are no mangroves at all, but instead, the immediate 

 shore is often steep and undercut. Evidently the extreme surface of the 



