470 FRANK SPRINGER 



of our former interpretation of the tegmen of the whole FlexibiHa 

 group. 



The two most instructive specimens are figured herewith (Plate 

 IV, Figs. I and 2), and another which gives a good general idea of 

 the disk (Plate IV, Fig. 3). Fig. i will be readily understood by the 

 simple statement that it is the ventral surface of the disk. In Fig. 2 

 we see the inner floor, or under side of the same structure, which has 

 been exposed by removing the basal, radial, brachial, and inter- 

 brachial plates. The disk is so pliant and frail that it is invariably 

 found more or less sunken down, and lying on the bottom of the 

 dorsal cavity. This is why it appears concave in Fig. i, and convex 

 in Fig. 2. The greater portion of the disk was clearly membranous, 

 with calcareous spicules and granules imbedded in it; except in the 

 oral center, and in the ambulacral regions, which are occupied by 

 rows of large elongate, alternating plates, more or less tumid exteriorly, 

 and keeled on the under surface. These converge in the middle into 

 a rounded, open mouth, which is surrounded by a pyramid of four 

 small, triangular oral plates, meeting a fifth large one on the posterior 

 side. This posterior oral is the most remarkable feature of the disk. 

 It is of enormous size relatively to the other structures, and is very 

 thick and heavy. It is preserved in nearly all the specimens in which 

 the ventral side was cleaned, but as it was usually found lying detached 

 in the bottom of the dorsal cavity, its identity was not recognized 

 until after I found it in place in this specimen (Fig. i). It is spade- 

 shaped, and the end toward the oral aperture is somewhat hollowed 

 to form one side of the mouth. Both the exterior and interior sur- 

 faces of this large plate are shown in the figures — the latter view 

 being at the upper part of Fig. 2. In Fig. i this plate seems to be 

 divided by a transverse suture, but I do not find this to be so in other 

 specimens. 



The four smaller oral plates are delicate, and they seem to vary 

 somewhat in shape in the different specimens — sometimes appearing 

 quite slender and elongate, and irregular in size. Much of this 

 irregularity, however, may be due to the accidents of fossiHzation, 

 producing such compression and displacement as might greatly 

 modify the shape and proportions of these delicate structures. In 

 the specimen represented by Fig. i, two of these plates are perfect 

 and undisturbed, the third somewhat flattened laterally, and the 



