REVIEWS 229 
forms an angle as in the case of a true bifurcation. The second pin- 
nule, which is somewhat smaller starts off from the fourth distichial on 
the opposite side as in the other species of the genus. All succeeding 
pinnules are small, and are given off alternately from successive joints. 
The oral plates have been the subject of much controversy, but their 
identification in the different groups is now pretty well established. 
According to Wachsmuth and Springer the orals are not always repre- 
sented in the adult. When present they surround the mouth or cover 
it. They may occupy the whole face of the vental disk or only its 
median portion. In the former case they rest upon the edges of the 
radials; in the latter against the perisome. In crinoids with a regular 
pentamerous symmetry they consist of five pieces interradially dis- 
posed, and form the center of the disk. When the symmetry is irreg- 
ular they are pushed more or less to the anterior side. The former con- 
dition prevails among recent crinoids; the latter is the general rule 
among paleozoic forms. When asymmetrical, the posterior oral by the 
encroachment of the anal plates, is pushed between the four others, so 
as to attain a more or less central position. The plate is generally 
larger than the other four. The orals in all groups in which they are 
represented consist of five pieces. There is no such thing as an oro- 
central plate, as some writers have supposed. In some instances the 
orals seem to be wholly or partly resorbed ; the former condition prob- 
bly is the case among the Camerata, the latter in certain species of 
the Fistulata. In regard to the Ambulacra it is now generally admit- 
ted that the aperture in the tegmen of palzozoic crinoids is not the oral 
opening but the anus, and the mouth is subtegminal forming the cen- 
ter of radiation, which, however, is not necessarily the geometrical cen- 
ter. The ambulacra follow the grooves along the ventral side of the 
arms, and extend from the tips of the pinnules to the mouth. Their 
inner ends are either exposed upon the disk, or covered wholly or in 
part by plates of the tegmen. The upper face of the ambulacra is 
occupied by the food grooves, which are roofed over by the covering 
plates and frequently are boarded by side pieces. In recent crinoids 
the covering plates are movable from the tips of the pinnules to the 
entrance to the mouth ; but in most palaozoic ones those of the disk 
are rigid, so far as known, often heavier, and larger than the interven- 
ing plates. The disk portions of the ambulacra in the Camerata, if 
tegminal form a component part of the tegmen, their plates being sut- 
urally connected with one another and with surrounding plates; those 
