22 The American Naturalist. [January, 
in the Cretaceous of New Jersey and the phosphate deposits of 
the Ashley River, S. C., seem to show that somewhat more 
favorable conditions for their preservation existed in these 
earlier times than at present, when bones appear to become 
quickly destroyed in the ocean, and only the most refractory 
substances, as enamel and very dense bone, resist the agencies 
of solution. Itseems altogether likely that the sedimentation 
must, at least at seasons or periods, have been rapid and con- 
siderable; that vast volumes of calcareous mud discharged 
into the cretaceous seas entrapped fish and reptile within the 
unchanging films and sheets of earthy matter. The wonder- 
fully preserved fish of the eocene in the Green River beds, ex- 
hibit instances of almost complete immobility, as if no motion 
had disturbed the fish since its death, no tide or current, and 
that it was quickly covered over by sediment. The fish and 
reptilian remains in Ohio, Illinois, Pennsylvania, Iowa and 
Missouri bear evidence of separation and rolling, the articula- 
tions being infrequently retained in place, the mouth parts — 
and skulls alone cohering together, though these are more 
commonly dismembered, while in complete uniformity with 
the experience of to-day, in many cases, teeth and spines are 
the sole representatives of these ancient denizens of the sea. 
Mechanical conditions under which marine vertebrate re- 
mains have been entombed very naturally affects the nature 
of their preservation. The sandy, coarse shore deposits which 
prevailed in the Catskill period—our equivalent of the Old 
Red Sandstone of Europe—was unfavorable for the cohesion 
of the fish which were enclosed in it, and the action of shore — 
waves and the agitation of the gravelly matrix broke them 
apart, and scattered over the shore surface the fragments of 
bones, scales and spines. On the other hand, the shallow sea 
basins wherein the Huron and Erie (now shown by Prof. 
Orton to be identical) shales were deposited, furnished a fine- 
grained impalpable carbonaceous silt in which occasionally 
the remains of the monstrous placoderm Dinicthys were en- 
tombed entire, and preserved with comparatively slight dislo- _ 
cation or injury. In the open sea of the Upper Helderberg 
and Carboniferous where conditions similar to our present seas _ 
ae 
