Review of Recent Geological Literature. 257 
the top. Then comes a sudden change; black shales, and black sheets 
of limestone, with silicious sand mixed with them ; sometimes beds of 
sandstone; but the order is various and different in separate sections 
a few miles apart. 
"It was in this black shale and limestone, with more or less sand- 
stone, that the new fauna arrived ; in the present case, a remarkable 
fauna for the locality. It included species which have never been 
seen before in the Mississippi valley, but are known in Nevada. It 
includes species which were never known before to occur so late in 
the series, but are common types in the preceding Devonian. The 
fauna points to some change by which communication was made with 
distant localities in the, then, sea basin." On the whole the Paleo- 
zoic faunas of northern Arkansas may be said to be the most not- 
able contribution to our knowledge of the Ozarks that has ever been 
made. c. R. k. 
What is an Echiuodcnii? by F. A. Bather. (Jour. London Coll. Sci. 
See. Vol. VIII, pp. 21-23, London, 1901.) 
An article which deserves more than passing notice by paleonto- 
logists is the admirable summary of our knowledge relating to the 
echinoderms, which Dr. Bather gave a short time ago in a lecture 
delivered before the City of London College Science Society. Coming 
from one who is not only a zoologist but who has had wide experience 
with fossil forms of life, the subject is viewed from the broadest pos- 
sible position. 
After considering briefly the characters by which an echinoderm 
may be distinguished from all other organisms, the three principal 
theories as to the origin and evolution of this group are summed up. 
These three theories are the ones which have had most influence upon 
morphologists. They are called the "Calycinal," the "Pentactsea." and 
the "Pelmatozoic" theories. The last of these seems now to be winning 
acceptance, but the other two cannot be dismissed without some words. 
The first of these theories, the Calycinal of Loven, Carpenter and 
Sladen, "asserts that certain skeletal elements, with a definite ar- 
rangement, forni the ground-plan of all echinoderms, which, therefore, 
are all descended from an ancestor with such composition of its skel- 
eton. The elements in question are the circlets of plates that are 
present in the theca or calyx of a typical crinoid, namely, the five orals 
around the mouth, the five radials from which the arms spring, the five 
basals below the radials, the five infrabasals occasionally found be- 
tween the basals and the stem, and finally a plate of more dubious 
nature — the dorsocentral." 
The "Pentactsea theory" of Semon crystallized the views of those 
who held that "the holothurians were the most primitive among echino- 
derms, and the family Synaptidas the most primitive among holothur- 
ians. Early in the life history of Synapta occurs a stage with five 
tentacles around the mouth, and into these pass canals from the water- 
ring, the radials canals to the body-wall making a subsequent, and only 
temporary appearance. Semon called this stage the Pentactula, and 
