Herbert L. Hawkins — Studies on the JEJchinoidea, etc. 203 



tubercles, with broad bosses, crenulated parapets (omitted in the 

 figure), and small mamelons, are set in areolae which extend very 

 little beyond them, and are arranged in an " hour-glass " plan on the 

 interambulacral, and with some regularity on the ambulacral, plates. 

 Abundantly scattered among these large tubercles, and roughly 

 encircling them, are small secondaries, similarly set in areolae, but 

 without defined mamelons and imperforate. These vary slightly in 

 size, but are remarkably uniform in character. Here and there 

 simple granules appear, without areolae; sometimes these are nearly 

 as large and prominent as the secondaries, but usually they are very 

 small. The fourth type is altogether different, and is somewhat 

 diagrammatically shown in Fig. 8, but an enlarged view (Fig. 6) makes 

 its characters clear. It consists of a series of pits of minute but very 

 constant size, encircled by faintly raised and ill-defined smooth 

 areas. These surrounding platforms are usually of about the same 

 diameter as the areolae of the secondary tubercles, but are rarely 

 quite circular, and never so distinct as either of the figures suggests. 

 Each of the structures might be compared to a shallow well with 

 a broad but low parapet. The encircled pits are present in about the 

 same-numbers as are the secondary tubercles, and, like them, may 

 enter into the scrobicular rings of the primaries, although the 

 majority are scattered without apparent order. None or the coronal 

 plates of the adapical surface, however small, is without them, and in 

 the ambulacra they may be found near or even in contact with the 

 peripodia. It is from these encircled pits that the small nipple- 

 shaped " spines " project in unworn specimens. 



One of the pits is shown, in plan and section, on PI. XIII, Fig. 6. 

 The pit is seen to be a roughly hemispherical concavity, comparable 

 with the negative of a mamelon. The encircling platform may 

 similarly be compared to a negative areola. 



As far as can be ascertained, the encircled pits occur in varying 

 numbers on all the coronal plates of the adapical surface of 

 C. alhogalerus. They are few and sparse on the young plates near 

 the apical system, and increase in number (with no corresponding 

 change in size) towards the ambitus, but I can find no trace of them 

 on the adoral surface. The plates there are much more coarsely and 

 closely granulate ; and, if the structures under consideration do exist 

 below the ambitus, they would seem to have become prominent instead 

 of re-entrant. ISTo pits occur on the plates of the apical system. 



The " spines " which spring from the encircled pits are remarkably 

 uniform in size and character over the whole adapical surface. Their 

 appearance when viewed in situ is precisely like that of the figures 

 given by Forbes (loc. cit., fig. 8). That author's remark that " they 

 are all perfectly smooth " is, if anything, an understatement. 

 The entire exposed surface of each " spine " is extraordinarily glossy, 

 outshining even the primary mamelons in polish. When it is 

 remembered that the radioles of this species have fluted, and even 

 fringed, shafts, and crenulated collars, the contrast between them 

 and the "spines" of the encircled pits is manifest. After many 

 failures, I have succeeded in examining the articulating surface of 

 the " spines ", both in solid and sectional view. They show an 



