20-A Herbert L. Hawkins — Studies on the Eel vinoidea, etc. 



interesting, but not surprising-, character. The greater part of the 

 area of the "surface of attachment" is quite smooth and flat, and 

 the whole surface is of considerably greater diameter than the top (or 

 distal end) of the " spine" owing to the splaying out of the lower* 

 part of the " shaft". In the middle of the somewhat irregular circle 

 of this surface there appears a roughly hemispherical knob, slightly 

 but distinctly projecting beyond the general plane (see PI. XIII, Fig. 7). 

 This knob is almost exactly of the same diameter as a pit, into which 

 it fits after the manner of the sutural knobs and sockets of Arlacia. 

 The smooth part of the lower surface of the " spines " seems to 

 correspond fairly accurately with the size and shape of the raised 

 parapets (or "negative areolae") around the pits. Such a mode of 

 articulation would allow of very limited movement ; indeed, the only 

 opportunity would be for rotation of the "spine" about its long 

 axis. There is no trace of muscular attachments, nor of perforation 

 for a ligament, either on the plate or on the "spine", but the 

 former, at least, may well have existed on the " negative areolae " as 

 they do on the normal ones. There is no indication of any syzygial 

 union between the " spine" and its pit. 



It seems, therefore, that an " encircled pit " represents a modified 

 secondary tubercle, whose mamelon and boss have become re-entrant 

 instead of projecting, supporting a glossy and short "spine" whose 

 acetabulum is convex instead of concave, and whose collar is 

 expanded to rest in contact with the raised areola. Whether the 

 " spine " is merely the proximal part of some such appendage as 

 a pedicellaria or not, cannot be determined. The surprising polish 

 of its entire outer surface would imply that, if any prolongation of 

 the structure existed, a region of soft, flexible tissue intervened 

 between that and the distal end of the " spine ". 



(b) Comparison with the Structures of D. ci/lindricus. 



The peculiar secondary tubercles of Discoides (referred to above) 

 are the only structures with which comparison is possible. Nothing 

 in the least like the " spiniferous pits " of C. albogalerus occurs, as far 

 as I am aware, in any other Echinoid, fossil or recent, excepting the 

 other species of the genus. In C. suhrotundus and C. rhotomagensis 

 they are present in similar numbers, and of the same character, as in 

 the Upper Chalk species ; so that they may be regarded as a generic, 

 or possibly a family, peculiarity. 



The specialized secondaries of D. cylindricus resemble the "pits " 

 of Conulus in many important respects. They are definitely 

 secondary tubercles in point of size and distribution, sharing with 

 them in the construction of the scrobicular circles. They have 

 areolae which are usually raised above the surrounding level of the 

 test, and which are never sunken like those of the normal tubercles 

 (see PI. XIII, Fig. 5). The mamelons are, it is true, convex, but they 

 are unusually small, and are set in a deep excavation above whose 

 rim they do not appreciably project. There is every reason to 

 believe that these mamelons supported small "spines", but these 

 would, owing to the exposed nature of their articulation, naturally 

 become detached by the same processes that remove the normal 



