76 



MADREPORARIA. 



Group VI.— BAHAMAS AND BERMUDA. 



62. Porites Bahamas 1. (P. Bahamm prima.) (PI. IV. figs. 2. 3, 4, 5, 6 ; PI. XVII. fig. 8.) 



[Nassau, coU. Melillo ; British Museum.] 



Description. — The corallum is convex, closely encrusting, and in all the central regions 

 covered with small rounded or steeply convex eminences, but laterally spreading out smooth 

 and thin. 



The calicles have deep conspicuous fossae, the largest 1 • 25 mm. on the rounded humps, the 

 smallest, often angular, are in the valleys. They become shallow as they near the thin edges. 

 The walls are usually very thick and round-topped in the central regions, but flat round the 

 edges. They consist of a flaky reticulum with rounded pores through the flakes ; the topmost 

 edges of the latter are frequently filamentous, but show the flakes in the lower levels. The 

 septa project a very little way as thin, insignificant and irregular processes from the edges of 

 the flakes, here and there with sKght lateral projections like the teeth of a saw. They 

 descend vertically downwards in very incomplete roiys, yet seen from above they separate deep, 

 short, round, interseptal loculi. The base of the fossa is filled with a large, flaky, columellar 

 tangle from the surface of which skeletal processes arise irregularly from the edges of the 

 flakes. Where the calicles are shallow, that is, round the edges of the stock, a few septa may 

 join with these columellar processes as bent irregular bars, but hardly any traces of the typical 

 symmetrical formula of Porites can be made out. Eound the edges there may be a width of 

 some 2 to 3 mm. of the flaky reticulum, with hardly any signs of calicles. 



The specimen a from Nassau, above described, is interesting because it is one of the 

 nearest approaches to a coenenchymatous West Indian Porites so far known (see Table IV. 

 p. 139). The walls thicken here and there to as much as 2 mm. In texture, they are 

 sometimes more flaky, at others more filamentous, see PI. IV. fig. 2. 



But the specimen is interesting also for another reason. It shows that small patches of 

 calicles with very thick walls are not always normal. There are two patches on this specimen 

 which show the calicles thus affected, in both cases, apparently by the proximity of foreign 

 organisms. PI. IV. fig. 4 shows a spot where the polyps of a Mussa (the skeleton of which 

 is still in situ) must have come into inconvenient and irritating proximity with those of the 

 Porites, with a result that the skeleton proliferates as tliere shown ; while again, PI. IV. fig. 3 

 shows a patch where the proximity of some foreign body is demonstrated ])y the sudden 

 flattening of the surface. This question was brought up once before, in the observation on 

 P. Guadalupe 6, with its two kinds of calicles. A certain amount of variation, due to natural 

 structural differences, is always to be expected, but it is well to know that very strikiug 

 difl"erences may be due to accidental interferences with growth. 



a. With a young colony e. Zool. Dept. 87. 4. 26. 8. 



