I 1 8 NEW YORK STATE MUSEUM 



while in the less saline waters of the shallow region they are thin and 

 small. 



The dolomite series from the top of the Rochester shale into the 

 Salina bed shows an irregularly increasing magnesian content and increas- 

 ing salinity. The occurrence of immense banks containing millions of the 

 extraordinarily ponderous Megalomus suggests that increasing salinity may 

 be an essential cause of its great size, for oysters are known to similarly 

 increase in size and thickness of shell in the deeper and more saline parts 

 of the sea. The corals, however, are considerably more sensitive to lack 

 of salt than to increase in salinity. They avoid the neighborhood of river 

 mouths but flourish luxuriantly in the Red sea, which receiving little fresh 

 water drainage, and having but restricted communication with the open sea, 

 is known to possess considerably higher salinity than the ocean without. 

 Altogether the Red sea with its greater salinity, extensive coral reefs and 

 abundant life seems an excellent portrayal of the conditions of the Guelph 

 sea. A complete inclosure of that body of water would repeat the condi- 

 tions that led to the formation of the Salina beds, with the exception that 

 the Salina sea still at times received much terrigenous detritus. 



Various writers have shown that once the optimum of salinity is passed 

 concentration of the brine produces disastrous effects on organisms. Dall 

 has shown that in the salt lagoons or salt pans of the Bahamas r the effect 

 of this concentration is shown in the diminished size and thin shells of the 

 mollusks and among the gastropods in a tendency to irregularity of coil 

 and erTacement of sculpture. Such extreme conditions may be conceived 

 to have led to the depauperation and actual extinction of the fauna of the 

 Salina stage. In this connection we may note one gastropod of the Guelph, 

 Loxoplocus solutus Whiteaves, which is unique among upper Siluric 

 gastropods in being a completely uncoiled Murchisonia, and which may 

 have received the impulse to its peculiar aberration from the gradually 

 increasing salinity of the water. 



■Mus. Comp. Zool. Bui. 1894. v. 25, no. 9, p. 113. 



