22 GEOLOGICAL MEMOIRS. 



over minute differences, does not consider ; and valuable as his la- 

 bours may be, and often are, with regard to such points of detail, 

 it is not thus that the true and philosophical naturalist is enabled to 

 advance his knowledge. 



Say has stated quite correctly that the Caryocrinite was possessed 

 of a round pedicle or stem [PL IV. fig. 1 (a)] attached to the 

 ground. Upon this stem four plates (^p) were developed as a pel- 

 vis ; upon these four (which are called basal), six lateral plates {q) 

 repose ; and six others again (r), called scapulars, rest upon these ; 

 while on this last upper series the arms (c, d) were planted. 



The general form is that of a large acorn, terminating below in a 

 blunt point, cylindrical in the middle, and closed at the top with a 

 flattened dome*. The four plates of the pelvis (p 1, jo 2), although 

 of unequal size, are notwithstanding so regularly constructed that 

 the pairs of adjacent ones are exactly alike, and exhibit a striking 

 symmetry of form. If however the two larger plates {pi) were 

 divided in the middle, they would be found to differ in no respect 

 from the smaller ones (jo 2) ; and in this case all the sides would be 

 alike, and instead of four, the pelvis would consist of six plates. This 

 separation of the larger plates is most clearly exhibited in the distri- 

 bution of the ambulacral pores on the surface, although the con- 

 fluence of the two plates — which is carried so far that the retiring 

 angle which ought to be formed on their upper side has become a 

 straight line (see Plate IV. fig. 1) — can only arise from the distri- 

 bution of the internal organs ; and the mouth is invariably found 

 upon the upper surface, exactly in the direction of the intersection 

 of the larger plates, this line of intersection being continued through 

 the middle of one of the lateral plates. The position of the mouth 

 — not placed in the centre, but on one side of the summit — is an 

 important point, and affects the lateral plate which conducts to it ; 

 since, although its form is not altered, this plate is notably deeper 

 towards the base than the similarly formed lateral plate corresponding 

 to it on the other side of the cup ; and in conformity with this mo- 

 dification, the scapular plate is also more extended, and descends 

 deeper than the similar plate on the opposite side (see figure). It 

 seems as if the excentrical position of the mouth had pushed down the 

 whole cup towards the base and pressed together the plates on this 

 side ; and, singularly enough, the side plates are found, in some ab- 

 normal instances, to be sometimes altogether pressed down, so that the 

 scapulars repose immediately on the basal plates, while on the other 

 side an additional plate is inserted between those regularly deve- 

 loped. This influence of the mouth upon the form and distribution 

 of the plates is universal in all the Crinoidea ; for where the basal 

 plates are not exactly similar in form and arrangement, there the 

 mouth is found invariably upon the side, an interradial or intercostal 

 plate conducting to it. In the Actinocrinites, for example, the base 

 of the cup consists of three small plates, of which two are consider- 

 ably larger than the third. Exactly where the two larger ones inter- 

 sect is placed that interradial so remarkable in the genus in question, 

 * See PL III. fig. 1, 2. 



