458 Mr. C. Wachsmuth on the Internal and 
are very thin, and that they rest but partly upon the consoli- 
dating plates, being thereby rendered insecure and liable to 
removal by any accident, even with very small force. More- 
over the arms of the Cyathocrinide are generally attached, 
and the ventral disk thus hidden from view. In specimens 
in which the arms are destroyed their destruction almost 
invariably involved that of the entire ventral side; and so 
delicate are these parts, that even when the arms are well 
preserved and so situated as to expose the dome, the plates are 
nearly always gone, or are found in a confused mass inside 
the calyx. 
I come now to another group, in which, on the basis of the 
summit-structure, such apparently diverse forms are included 
that I am under the necessity, very unwillingly, of making a 
name for it. It includes the families Actinocrinide, Platy- 
.erinidee, Rhodocrinide, Melocrinide, and the genera Schizo- 
ertnus, Dimerocrinus, and Macrostylocrinus, which Roemer 
has ranged among the Cyathocrinide ; and I call it provi- 
sionally the Spheroide, from the form of the calyx, which is 
generally somewhat spherical. This large group, embracing 
over one hundred genera, and ranging from the base of the 
Silurian to the top of the Subcarboniferous, is capable of 
accurate definition, is easily distinguished, and, fortunately, the 
summit is very commonly found well preserved in most of 
the genera. 
The summit is composed of heavy, frequently nodose plates, 
closely cemented together so as to form a free arch (not sup- 
ported by consolidating plates), which rests like a hemisphere 
upon the dorsal cup. The plates of the summit, which at first 
sight exhibit great apparent diversity, are arranged through- 
out upon one and the same fundamental plan. Beginning 
with genera that have but few vault-pieces, we find in them 
the median portion occupied by one large centre plate, sur- 
rounded by six others—four large ones of equal size, and two 
smaller ones. The four large plates join laterally, and are 
often placed directly to the centre piece. In very large species 
and sometimes in very old specimens the plates are separated 
by small polygonal pieces, but easily recognized by their size. 
‘T'wo of the four plates lie above the interradial series adjoining 
the anterior ray ; the two others, one at each side, are placed 
between the two lateral rays. The two smaller plates are 
separated from each other by anal plates or by the proboscis. 
These seven pieces, which I will call the “ apical plates,” are 
easily recognized by their greater prominence and size in 
species with comparatively few summit-plates and a lateral 
anal aperture; but their identification is more difficult in 
