NIAGARA GROUP. 
223 
the same form ; while in Caryocrinus the number is six, two of the four pelvic plates having 
the form of double plates, above which are six costal plates succeeded by sbc scapulars, and the 
mouth is composed of six plates. I have previously shown that the tripetaloid pelvis is a se¬ 
condary or derivative type from the pentapetalous base, or at least that it follows the latter in 
point of time. Those crinoids with tripetalous bases, besides showing two of their plates to 
be double, have always in their higher structure the number five, or some multiple of that 
number, in the plates of the body and number of arms. 
In a single species of Crinoid known to me in the Hamilton group, the base is composed of 
six distinct plates, showing that this feature is not alone confined to Caryocrinus. This struc¬ 
ture is, however, unquestionably very rare ; probably commencing in the Caryocrinus, and 
appearing in other forms of this structure modified, in later geological periods. The Cystidea of 
Yon Buch, properly so called, though having four pelvic plates, do not possess this easy sub¬ 
division into six; neither do the succeeding series of plates present characters analogous to 
those of Caryocrinus. 
Fig. 1 a. An individual of this species, of medium size, showing a portion of the column below. 
The actual position of the mouth is clearly shown. 
Fig. 1 b. A larger individual, destitute of a column. 
Fig. 1 c. The base of the same or a similar specimen, showing the form and arrangement of the 
pelvic plates, and succeeding costal plates. 
Fig. 1 d. A specimen of large size, having a portion of the column still attached. This individual 
shows the anomalous feature of having one of the pentagonal costal plates truncated 
above, by which it becomes hexagonal. This is shown in the left-hand costal plate 
in view, which is contiguous to the heptagonal plate on the right, as seen in the 
same figure, and to the left of the hexagonal costal below the mouth. This peculiar 
feature introduces a supernumerary scapular or interscapular, making nine in the 
higher series; thus increasing the breadth at this point, and giving space for the 
attachment of another pair of arms. The one double row of pores that usually extends 
to the higher angle in this plate, has become two double rows from their origin at 
the centre of the plate ; but below this point, the whole crinoid has the same character 
in every respect as other specimens. 
Fig. 1 e. A small specimen, slightly more elongated than usual, where the pores are less strongly 
developed than in many other specimens, and the marking of the surface very mi¬ 
nutely granulate in regular concentric lines, without tubercles or ridges of any kind. 
Fig. 1 /. A specimen of about the same size, having a character precisely the reverse of the 
preceding. The surfaces of the plates are covered with strong tubercles, and ridges 
diverging from the centre to the angles, at the bases of which the pores are nearly 
buried. 
These opposite characters, therefore, do not appear to depend upon the age of the 
individual, but upon other circumstances. I have another specimen, of no greater 
length than the preceding, but proportionally wider, where the ridges and tubercles 
are even stronger than upon this one. 
Fig. 1 g. A young specimen with a part of the column attached. 
