REVIEWS 225 
three and two pieces, or all five plates may be anchylosed, so as to form 
a single piece. Among the Camerata five basals are restricted to the 
Lower Silurian forms, four basals to those from the Upper Silurian and 
Devonian, three to those from Upper Silurian to the Lower Carbonif- 
erous, and two in only some forms from the Carboniferous. The 
diminution in number takes place in geological succession, and is the 
result of fusion of two or more of the original five plates, as is clearly 
seen in genera without an anal plate between the radials. In forms, 
however, in which an anal plate is represented and the basal disk is 
consequently changed from a pentagonal to hexagonal shape the case 
is somewhat more complicated, for a bisection of the plates in the 
hexagonal base would produce six basals instead of five. The intro- 
duction of the anal among all the monocyclic groups is accompanied 
with an increase in the size of one of the basals, there being no special 
basi-anal plate. In the tripartite base, the smaller plate— always the 
left antero-lateral one—doubles its size. In the quadripartite base the 
increase is towards the right of the posterior plate ; while in the bisected 
base in which the left postero-lateral basal, the antero-lateral, and the 
anterior one are fused, the two plates of the opposite side increase in 
size sO as to correspond with the compound plate to the left. In 
dicyclic crinoids the introduction of the anal does not affect the 
arrangement of the infrabasals, and only slightly the form of the basals. 
In species with three infrabasals, one of the plates is always only one- 
half the size of the other two. This ossicle is, in the Ichthyocrinidz 
and comatula larva directed toward the right posterior radial; but in 
the Inadunata its position is not constant. The basals of dicyclic 
crinoids are but little affected by the presence of the anal, only the 
upper angle of the posterior plate being slightly truncated. 
When it was discovered several years ago, by Wachsmuth and Springer 
that among Palzocrinidz there is a regular alternation of the successive 
parts below the radials it was also found that the orientation of the 
stem in the monocyclic groups is reversed in dicyclic forms. In the 
former the sharp outer angles of the stem are radial; in the latter 
interradial. The central canal and the cirri are interradial in the first 
mentioned forms, but radial in others. The lawis, however, applicable 
to its full extent only in species with pentangular or pentapartite stems, 
but it is concluded from analogy that the circular stem, wherever it 
occurs is also practically interradial in dicyclic crinoids and radial in 
monocyclic ones. However, on applying the rule to mesozoic and later 
