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 six 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 

 Von 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 J. 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. \ 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 y. 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 ^. A young specimen with a part of the column attached. 



