218 



PALiEONTOLOGY OF NEW-YORK. 



It is only in a comparatively small number of specimens, that the details of the beautiful struc- 

 ture are clearly shown. 



The pelvic plates are four as described, two of them pentagonal and the other two hexagonal, 

 the vertical sides being equal in all. The upper side of the hexagonal plates is nearly straight, or 

 slightly concave ; and they have the form and surface characters of two plates, like the penta- 

 gonal ones, joined together at their lateral margins. The upper margin between the prominent 

 angles, instead of presenting sloping sides, to a central retiring angle, is extended in a nearly 

 straight or slightly concave line. 



This feature is more conspicuous when we examine the pores, which extend in double rows 

 from the base of the pelvic plates to the prominent angles of each one, the larger plates having 

 two of these double rows of pores. The six costal plates consist of three pairs of similar form ; 

 two of these pairs alternating, and one having their bases opposite, giving, with the basal plates, 

 a beautiful symmetry of form to this part of the body. Rarely, there is some deviation in the 

 form of one or more of the costal plates ; but this is usually at the summit of the plate, scarcely 

 affecting the form below that point. The scapular plates, with rare exceptions, are triangular 

 below, having two sloping sides resting on the upper sloping edges of the costal plates : their 

 lateral margins are nearly vertical, which is likewise true of the interscapular plates, which 

 have a straight base. The upper margins of all these are more or less irregular, from the in- 

 sertion of the plates around the base of the arms ; they are usually, however, two-sided on the 

 upper margin, sometimes three-sided, and sometimes truncated. The summit is composed of an 

 unequal number of plates, the central one being largest, heptagonal, and surrounded on six of 

 its longer sides by the same number of large plates ; while the short seventh side adjoins a 

 smaller plate, which sustains, in part, the plates of the mouth, which organ is always near the 

 margin of the summit. 



If a line be traced from the base through the division between the larger pelvic plates, 

 passing through the centre of one of the hexagonal costal plates, and thence to the summit, 

 it leaves the mouth a little to the right, as regards the observer, or, in reference to the animal, 

 a little on the left side of this line*. 



There is a wider separation of the arms at the point where the mouth occurs, and in conse- 

 quence a slight depression producing a flattening upon that side ; while on the side directly 

 opposite, the bases of the arms are more prominent. This gives a kind of triangular form to 

 the summit, the mouth being placed in the centre of the base ; and a line drawn, as before 

 mentioned, through or near the mouth and the apex of this triangle, will divide the fossil in 

 two equal portions, the component parts of each having the same form and relative position. 

 This bilateral arrangement of the parts is very conspicuous in many well preserved specimens. 



column or an arm. The recent excavations for the enlargement of the Erie Canal, though much less in extent, have 

 enabled us to procure more perfect specimens of this crinoid, and greatly to extend the list of genera and species of 

 this family. 



*M. Von Buch says the mouth is directly in this line ; but in more than one hundred specimens examined, it is 

 not 80, but more nearly over the centre of the next scapular plate. The difference, however, is of small importance, 

 but shows conclusively that this organ is always nearly in the line dividing the animal into two equal parts. 



