24 PLATYCRINID.9E. PLATYCRINITES. 



The Abdominal, or coronal plates. — Immediately over each of the excavations, for 

 the insertion of the rays, are three small smooth plates, the centre one of which is 

 pentagonal, but the other two approach to the quadrangular form. These are succeeded 

 by five large and two small cajjital plates, which crown and cover in the vertex. The 

 largest of these plates is heptagonal, and its place is a little removed fi-om the centre ; 

 the four accessaries or those next in size, are also mostly heptagonal, but irregularly so, 

 and they sometimes even vary so much in form as to become hexagonal. These four 

 plates fit on to an equal number of the edges of the larger and more central plate. On 

 two other of its edges are the two lesser plates, which are irregidar pentagons. The 

 centre of each of these seven plates is sufficiently produced to form an elongated pomted 

 tubercle. 



The Mouth. — Between the two lesser plates is the lateral oral aperture, but the exact 

 form of the mouth is unascertained, as the valves, or plated integument which protected 

 it, have been removed from the specimen from which our figure is taken. From the 

 form and arrangement of the plates around the oral aperture, it is e%ident that the 

 mouth was incapable of extensive protrusion, and shoidd more instructive specimens be 

 obtained, this ojiinion we venture to believe will be found correct. 



The RAYs.^Number unknown. The excavations in the ray bearing plates are five 

 in number, as in the former species, but they are larger, and from the great size of the 

 meso-plates, the rays themselves were probably thicker than in the typical species. 



The size of the meso-plates, we imagine always bear a certain relation to the size of 

 the rays themselves. Thus in those species where the basal joints of the arms are 

 massive, the meso-plates are of considerable size, but when the former are delicately 

 formed, the latter are projjortionally smaller. Reference to our second plate will better 

 exemplify our meaning ; and though in the absence of the rays themselves we can offer 

 but negative evidence in support of these views, yet an extensive examination of other 

 species adds to the probability that our conjectiu-es are in the main correct. 



The orifices for the passage of the muscles which moved the rays will be found much 

 larger in the P. mucronatus, than in the P. antheliontes, as will the meso-plates of the 

 former be found more than double the size of those of the latter species. This difi'erence 

 in structure we consider as indicative of a difference in the size of the rays ; and in the 

 alisence of the rays themselves, it will be found a convenient criterion by which to form 

 a comparative estimate of the size or number of the absent members. 



The Tentacula. — Unknown. 



The Column. — The imbedded portion of a column Ijing alongside the specimen 



