H. L. Hawkins — Tuberculation of Holectypoida. 451 



from apex to peristome. It was only after persistently recurrent 

 examination, and the drafting of frequent, often varying, diagrams of 

 each specimen, that the system became manifest. One reason for its 

 obscurity lies in the fact that the tubercles of the median series are 

 placed very near to the adoral margins of the plates, and so lose the 

 prominence which comes of isolation. The other, and most important, 

 reason, is the order of position of the tubercles on the interradial 

 tracts of the plates. An examination of one interambulacral plate of 

 this species shows the tubercles to be arranged in two alternating 

 horizontal lines, reaching from the adradial to the interradial sutures, 

 and so closely packed that there is little room for granules between 

 them. This is due to the presence of oblique pairs of tubercles on 

 the interradial, as well as on the adradial, tracts ; the adoral situation 

 of the central series bringing those tubercles into a horizontal line 

 with the lower members of the zigzag couples. The importance of 

 this arrangement lies in the fact that the interradial tract, instead 

 of being relatively poor in tubercles, becomes almost as thickly covered 

 with them as the adradial portion. This is in striking contrast to the 

 features of all the previously described forms, but affords a link with 

 the genus next to be described. 



8. Conuxus albogaleeus, Leske. 

 (a) The Structure of the Tubercles. (Fig. 1, G.) 

 In this genus the difference in the proportionate diameter of the 

 tubercles of the upper and lower portions of the test is not so 

 pronounced as in Discoidea, but more so than in Anorthopygus. The 

 retention of a well-marked scrobicule in the adapically situated 

 tubercles, and the reduction of the ' granules ' to the nature of pits, 

 renders the upper ones very obvious, in spite of their usually low 

 relief. The scrobicule is circular, quite deeply excavate in adoral 

 tubercles, and well-marked, though shallow, in those of the adapical 

 surface. The boss rises almost directly from the scrobicular circle 

 with a sharply convex outline. On the adoral surface the boss rarely 

 exceeds in height the depth of the scrobicule — adapically it usually 

 projects slightly above the ring. The platform is broad, without 

 a marked parapet, and quite smooth. The mamelon is small and has 

 a deep central foramen. 



(b) The Arrangement of the Tubercles. (Fig. 2, G.) 



Although considerably more complex in order of sequence, the 

 tubercle series in Conulus are far more readily traceable than in 

 Anorthopygus. This is due to the more nearly central position (in 

 a vertical sense) of the tubercles of the main series. Although these 

 are often smaller than the lateral tubercles, and never larger, their 

 isolation and position render them useful indices of the boundary 

 between the adradial and interradial tracts. Except in very young 

 forms of this species (and in moderately small specimens of C. sub- 

 rotundus) there are rarely more than two plates at the apex which bear 

 only one tubercle. There is thus a marked acceleration in the rate 

 of appearance of the tubercles — even more than in the case of 

 Anorthopygus. As in that genus, the interradial series of tubercles 



