A FISH BRAIN FROM THE COAL MEASURES 137 
II. FOSSILIZATION OF FISH BRAINS 
The soft parts of fossil vertebrates have been replaced in a 
few cases by slow infiltration of mineral substances just as skele- 
tal structures are fossilized by replacement of osseous sub-_ 
stance by minerals. Such has been the apparent method of 
preservation of the small brains from the Coal Measures, de- 
scribed in this paper, as well as the brains of Rhadinichthys 
described by Eastman (’08) from the Mississippian of Kentucky. 
In regard to the manner of preservation of these brains, East- 
man (’08, p. 269) says: 
* * * It is evident that we have here to deal with a veritable 
brain-structure the substance of which became transformed into 
calcium phosphate before decomposition could set in, and whose walls 
in consequence are scarcely shrunken. This view is further con- 
firmed by the presence of nerve fibers and blood vessels, slightly en- 
larged in some cases, it is true, by the segregation of mineral matter, 
but coinciding in position with altogether similar nervous and vascular 
structures in modern ganoids and bony fishes. 
The precise manner of formation and preservation of the fossil 
fish brains has never been determined, so far as I am aware. 
Much work has been done, it is true, on the formation of phos- 
phatic nodules (noted by Eastman, 1908, footnote, p. 266) 
and it is possible that the brain substance has been replaced by 
a slow infiltration of calctum phosphate; although such a replace- 
ment is very difficult of comprehension in view of the chemical 
analysis of the brain, which, as ascertained by Waldemar Koch? 
for the human brain, contains a very high percentage of water, 
and a smaller but appreciable amount of soluble substances. 
The amount of water in the brain is much greater among the 
lower vertebrates. Doctor Donaldson writes me that his tables 
show 84 to 85 per cent for the proportion of water in the brain 
of the common frog, Rana pipiens; 76 to 78 per cent in the entire 
brain of the summer flounder at Woods Hole. Observations 
on the brain of the albino rat show 88 per cent at birth and 78 
per cent at maturity. George G. Scott® gives a range of 75.5 
to 83 per cent according to age in the selachian, Mustelus canis. 
5 Amer. Journ. Physiol., vol. 11, no. 3, p. 326. 
6 Scott, George G. 19138. Annals N. Y. Acad. Sci., vol. 28, p. 52. 
