REVIEWS 231 
tegmen consists of but one set of ossicles and that the plates are sutur- 
ally connected and solid on the outside, but perforated and vesicular 
within. The condition of the ambulacra in camerate crinoids, whether 
tegminal or subtegminal, does not represent an essential structural feat- 
ure, but is a natural consequence of differences in the form and con- 
struction of the tegmen in the respective groups and as such cannot be 
of much value from a morphological or classificatory point of view. 
Subtegminal ambulacra, as a rule, are most prevalent in species with 
high dome and bulging arm basis; while forms with a flat or depressed 
ventral surface generally have tegminal ambulacra. The two styles 
occur side by side among species of the same genus, and there exist 
all possible transition forms between the two extremes, z. ¢., specimens 
in which the ambulacraare subtegminal at the median portions of the disk, 
and tegminal near the periphery. By comparing the younger individ 
uals with the older, it appears that the covering of the ambulacra is pro- 
duced in the growing animal by the gradual extension of the interambu- 
lacral areas along the lines of the ambulacra, either completely covering 
them, or leaving the portions next to the arm basis exposed. The 
ambulacra of the Camerata, therefore, are covered not by an element 
unrepresented in other groups, but by small superimposed plates passing 
out from the disk proper. ‘These plates were quite small in the Silurian 
species, but change essentially until in the Carboniferous they frequently 
attain the large size and rigidness of the other plates in the tegmen. 
As to the closure of the mouth, it is now believed that it was subsequent 
to the introduction of the anal plate, by means of which the posterior 
oral was pushed in between the four others so as to close the opening. 
The interbrachials and interambulacrals, in most of the Camerata, 
pass insensibly into one another, there being no line of demarkation by 
which they may be separated, except that produced by the arms, and it 
is difficult to understand how these plates can be distinct structures as 
is generally supposed. That their morphological relations are very 
close is conclusively shown by the fact that the very same plates which 
in the Actinocrinide and Batocrinide are strictly interbrachial, are in 
the Platycrinide and Hexacrindidz partly interbrachial and partly 
interambulacral, and in the Cyathocrinide exclusively interambulacral. 
That the plates of the two hemispheres occasionally are interrupted, 
notably in Batocrinus, Catocrinus and Strocrinus, is readily explained 
by the large increase that here takes place in the number of arms, which 
prevents the development of interbrachials around the arm bases. 
