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THE AMERICAN NATURALIST [Vol. XLII 



entire sea for a large distance from their mouths, and 

 thus render crinoid life impossible. 



Within the tropics, particularly in the East Indies, 

 very large comatulids belonging to the Tropiometridse, the 

 Zygometridae, the Himerometridas, and the Comasteridae 

 occur abundantly in very shallow water, often just below 

 the low tide mark; moreover, they decrease in size with 

 depth. This would appear to directly contradict the 

 conclusion reached in the case of Anfc 'don bifida, but in 

 reality the problem is an entirely different one. Within 

 the tropics the intense scorching sunlight causes rapid 

 evaporation from the surface of the sea, especially where 

 the water is shallow, and a consequent mortality among 

 the more delicate organisms. The beaches and rocky 

 shores, at low tide, warm up, to be covered again at high 

 tide with comparatively cool water, full of organisms 

 unable to stand a great difference in temperature, which 

 are consequently killed and swept back into the sea, to 

 fall just beyond the low tide mark. Periods of glaring 

 sunshine are relieved by torrential rains, which are just 

 as fatal to pelagic life through the sudden lowering in 

 the density of the surface water. Thus it is evident that 

 within the tropics the sublittoral zone and the sea bottom 

 near the shore line offer the maximum food supply for 

 the crinoids, and explain the occurrence in such localities 

 of members of these four families of very large size. 



But torrential rains are associated with mountainous 

 districts; a glance at the distribution of the species of 

 these four families shows that all of the large species and 

 practically all of the small ones occur exclusively about 

 mountainous islands or near high mainland, and they are 

 particularly abundant along the shores of the larger East 

 Indian Islands. On isolated coral reefs and about the 

 shores of low coral islands where, owing to the very low 

 altitude of what little land there is the rainfall is very 

 small, these large littoral crinoids are quite absent. 



The comatulids are divided into two great groups, one 

 with triangular pinnules and small eggs, the Thalas- 



