MORPHOLOGICAL PART. 109 
crinidx, the covering pieces are generally exposed in the calyx, while in the 
Actinocrinide they are as a rule hidden from view. But occasionally the 
opposite is the case, and even among species of the same genus.  Ae/inocrinus 
stellaris, from Belgium, has well defined covering pieces passing out from the 
outer edges of the orals; while almost every other species of Acténocrinus has in 
place of them radial dome plates of the first, second, and third order, accord- 
ing to the number of bifurcations in the calyx. Physetocrinus, which is but a 
modified Aciinocrinus, has in some species orals and radial dome plates ; while 
in others the whole ventral surface is covered with small, irregular pieces. 
In Actinocrinus multiradiatus, the entire tegmen consists of only a few unusu- 
aly large interradial plates, which interlock with those of the dorsal cup. 
But the interradials in most of the Actinocrinide pass insensibly into the 
tegmen, there being no dividing line; while in Buatocrinus generally, but 
not always, the interradials of the dorsal side are distinctly separated from 
those of the ventral side by the arching brachials, —a structure which led us 
at first to suppose that the plates of the two hemispheres were morphologi- 
cally distinct. 
Similar variations occur among the plates of the tegmen in the Platy- 
crinide and Hexacrinidx. In some of their species the pavement is made up 
entirely of massive plates, in others of comparatively thin pieces; while in 
still others the ventral surface is occupied almost exclusively by the orals. 
In both groups it is absolutely impossible to draw a dividing line between 
interbrachials and interambulacrals. The plates constituting the first row, 
which generally consist of three pieces, are peripheral and partly interam- 
bulacral, and those of the succeeding rows strictly ventral. The plates of the 
second and higher rows, when such are present, interlock with those of the 
first row, like the interradial plates of the dorsal eup in an Aetinocrinus. 
The condition of the ventral pavement in the Melocrinidx, Rhodocrinide, 
and Thysanocrinide is similar to that in the Actinocrinide. Their lower 
interbrachials are definitely arranged, and there is no line of demarkation 
between the plates of the two hemispheres, except that produced by the 
arms, which pass out between them. In the Reteocrinide, as in most of the 
other Silurian Camerata, the whole ventral surface is covered by minute, 
irregular pieces, and similar plates, with a few somewhat larger ones scattered 
among them, are interposed between the rays from the basals up. 
It seems to us perfectly clear from the structure that all interradial and 
interaxillary plates, not only in the Camerata, but also in all recent and fossil 
