34 JOHN M. CLARKE 



WHERE DO THESE DEVONIAN SPONGES COME FROM AND WHAT 

 WAS THEIR ANCESTRY? 



The answer to the first query may take this form: Between the 

 species in the dark shale of the early Ordovician (Levis shales) and 

 this invasion in the late Devonian, there is but a single recorded 

 species which would seem safely placed among the Dictyosponges, 

 viz., Dictyospongia danhyi McCoy from the Upper Ludlow (Silu- 

 rian) of Westmoreland. We have referred to a similar occurrence in 

 the Silurian of New York. It is quite possible that these were but 

 derelicts tossed shoreward. The striking hexactinellids described 

 by Dawson from the Levis shales seem to have for the most part the 

 simple obconical foundation with special developments of spicular 

 tufts which indicate that up to this time there had been no wide 

 departure from the simple type of Cyathospongia which is repeated 

 in the genus Dictyospongia. The other differentials of the Dictyo- 

 spongida do not appear.^ 



The dark shales in which those early species (Ordovician and 

 Cambrian) are preserved indicate a greater depth of water than 

 do the Devonian colonies. We may therefore think of them as 

 having invaded the deeper epicontinental seas from the much deeper 

 waters of the continental edge at a time when the way was freely 

 open to the margins of the platform. If they were traveling in 

 toward ever-shallowing water there would or should be remains 

 of them in the black shales and the sands of the interval deposits. 

 There are none, and the fact constrains us to think that, instead of 

 traveling into shallow waters, they were moving back to the deeper 

 waters, where, concealed from the accessible rock records, they were 

 working out their evolution. Then some impulse which may not 

 be defined^ drove them into the shallow epicontinental waters, 



^ It is presumed, but not proven, that these early hexactinellids were Lyssacine, 

 that is, had the parenchymous tissue filled with detached spicules as contrasted to 

 the fused parenchymalia of the Dictyonina. 



^ Austin H. Clark, writing of causes of marine migrations, says: "Internal specific 

 pressure due to enormous increase in the number of individuals within a species operates 

 not only to cause a species to colonize bathjonetrically undesirable locations or unnatu- 

 rally cold and uncongenial regions such as the polar seas but also to force species into 

 small localized areas." (Quoted by Ruedemann, Paleontology of Arrested Evolution, 

 p. 128, 1918.) 



