GULF OF MEXICO PORITES. 69 



gradually down the sides, with a central tubercle as its most persistent element, the tangle 

 gets more and more condensed and conspicuous, until, near the edge of the colony, it fills up 

 the base of the fossa at a moderate depth. The interseptal loculi show irregularity in sizes, 

 some being very large. 



In the only exposed section, tlie trabecule are well developed ; they are fairly far apart 

 and joined by horizontal pieces, together making a rectangular network. 



Besides this, there is another massive form from Belize, and according to the view of 

 those who consider all the massive forms to belong to one or two species, this second would 

 have to be classed with that just described. But the first thing we want to know is, what are 

 the facts ? and here are two massive forms from the same locality, yet differing in growth-form, 

 in surface-markings and in tlie structure of tlie calicles. It is impossible to deny that they 

 may ultimately prove to be of the same "species," but until tliis is established, we have nothing 

 to do but follow the facts, and those are confined to what we can discover from a comparison of 

 the two on the lines laid down above. For the striking differences they present, see the 

 observations under the next heading. 



a. ZooL Dept. 57. 12. 16. 5. 



55. Porites Belize 3. (F. Belizei tertia.) (PI. I. fig. 7 ; PI. XVII. fig. 7a.) 

 [Belize, coll. J. Smith ; British Museum.] 



Description. — The corallum, a large, long, oval, massive form, appearing as if built up of a 

 number of convex oval plates, the margins of the plates Iianging down all round to the 

 substratum, here free, there closely encrusting. Some continuity of growth, however, doubtless 

 persists in the central region, but on this puzzling growth problem see § IV. of the Introduction, 

 p. 20. Tlie whole stock is about 12 cm. liigh by 20 cm. across by 25 cm. long. The surface is 

 thrown up into so many nearly separate, bluntly angular waves of all sizes with smooth 

 slightly convex spaces between. These waves are conspicuous and about 1 • 5 cm. high, and 

 it is noticeable that they repeat the shape of the whole stock (see p. 15). 



The calicles show as circular pits. The fosste, which can alone * be measured, range from 

 1 mm. downwaixls, on the tops or slopes of the waves, to minute angular breaks in some of the 

 valleys. The walls consist everywhere of an elegant, open, angular, filamentous reticulum, the 

 free ends of which form septal points and otherwise toss freely in different directions. There is 

 no trace of symmetry in tliis network, and even when in the valleys it is thinned down, it 

 shows hardly any tendency to return to the typical zigzag wall-thread. These walls are 

 thickest, up to 1 mm., on the waves, and may be eitlier fairly smooth and flat or else surge up ; 

 they are thinnest and most straggling in the valleys. They give the whole surface a soft 

 woolly aspect. The septa are short, very irregular points which descend to the rather open, 



* This therefore supplies us with no stire basis for comparison with other forms ; the ordinary 

 measurements are from wall-ridge to wall-ridge. 



