24? GEOLOGICAL MEMOIRS. 



clear that these pairs of arms and their protrusion have distorted 

 the upper surface of the triangle. Without them the scapular plates 

 would have been continued without interruption to the middle point 

 or apex, and instead of lateral plates, as they now are, would have 

 become vertical plates, as in Hemicosmites. So essential is the 

 change that has been effected throughout by the introduction of 

 these arms ! The number of arms is nine or six, according as those 

 proceeding from the angles are to be considered in each case as two 

 arms, or (as in Platycrinite and allied forms) as each consisting of 

 one arm which has branched off immediately into a pair ; but with 

 regard to this point we have at present no knowledge, since not one 

 of the specimens hitherto described exhibits the arms at their attach- 

 ment. 



This very remarkable triangular form of the upper surface of the 

 Caryocrinites, marking like a roof the general contour of the cup, 

 determines also the form and the arrangement of all the separate 

 parts ; and of these we may first mention the mouth, which is placed 

 on the edge, on the right half of one of the sides of the triangular 

 summit. 



The mouth is a large opening, its diameter being generally at least a 

 fourth part of the whole breadth of the upper surface, and it is closed 

 by five, or, more commonly, six convex valves, which form a small 

 cone, and are moveable, as if on hinges, being set in little grooves 

 or depressions round the edge of the mouth. Of the other two sides 

 of the triangle, the central part is exactly determined by the middle 

 of the heptagonal plates of the side ; and hence these are readily 

 made out in cases where from injury or the presence of extraneous 

 matter they might otherwise be concealed. The middle point of 

 the triangle, and therefore of the whole upper surface, is marked by 

 a small, very delicate, little elevated, and sharply defined plate, sur- 

 rounded by others somewhat smaller, but of exactly similar shape, 

 arranged in a circular group, and with the greatest regularity. The 

 central plate would be a complete and regular hexagon, as the sur- 

 rounding ones really are, were it not for the interference of the 

 mouth, one of the very small plates surrounding which is pushed in 

 on that side between the regular series of plates circularly arranged, 

 so that the number of these latter is increased to seven, and the cen- 

 tral plate itself receives an addition of a small seventh side. This 

 arrangement of the summit, as well-defined as it is elegant, is com- 

 mon to all those genera of Crinoidea in which the mouth is excen- 

 trical. It is strikingly exhibited in all the species of Plaiycrinus, 

 since in most of these the ranges of central plates terminate in 

 pointed summits, and like a circle of little turrets they surround a 

 higher central tower. In Rhodocrinites also (described by Phillips 

 as Gilbertsocrinite in the Annals of Nat. Hist. vol. xi. p. 202) this 

 peculiarity of arrangement may be well observed, although the plates 

 themselves are but little pointed. In the drawing by Phillips, 

 which in other respects is good, this arrangement is not shown, and 

 the upper surface exhibits only confusion and irregularity, but it is 

 admirably represented by M. Miiller {Pentacrinus, tab. 6. f. 1. c). 



