Herbert L. Hawkins — Studies on the Echinoidea, etc. 205 



radioles. Thus the only serious difference that exists between these 

 peculiar secondaries and the "pits" of Conulus is the presence of 

 convex rnamelons ; but these are far less prominent than the normal 

 ones of the same specimens. It may be fairly confidently suggested 

 that the specialized secondaries of Discoides show an early stage in 

 the evolution of the "pits" of Conulus, the areolae having already 

 risen, but the tubercles being only partly reduced. The apparent 

 absence of the "sunken tubercles" of Discoides from the species of 

 Conulus, and the almost certain absence of "encircled pits" from 

 Pyrina, invite speculation as to the relationships of these three 

 genera into which it would be premature to enter in this paper. 

 It is sufficient for the present purpose to point out that, as regards 

 these curious sui-face features, Discoides possesses both types, one 

 of which occurs in Pyrina and the other in Conulus, two genera that 

 are far in advance of Discoides on the path of true " Irregularity ". 



4. Summary. 



Two sets of specialized tubercles are described, " sunken tubercles " 

 and "negative secondaries". Both types occur in Discoides 

 cylindricus, but the former type only is fully developed there. These 

 " sunken tubercles " are believed to occur in Pyrina also, and are 

 further correlated with the " glassy tubercles " of JEchinonetis. They 

 also occur in the earlier Holectypoida. The " negative secondaries " 

 are found in their fullest specialization in Conulus, and there support 

 the short, closely articulated, glossy "spines" that are familiar to 

 all collectors who work in the Upper Chalk. There is thus an 

 indication that there may be two distinct lines of descent from 

 Discoides (omitting the obvious" sequence to the Clypeastroids), in 

 one of which the changes led to the recent Eclunoneus through 

 Pyrina, while iu the other Conulus marks the first stage. No 

 suggestion as to the physiological value of the two series of structures 

 can be advanced. 



EXPLANATION OP PLATE XIII. 

 Fig. 



1. Discoides cylindricus. Adapical portion of corona, x 5. 



2. ,, ,, Plates midway between apex and ambitus. x 5. 



3. ,, ,, A sunken tubercle in plan and section, x c. 30. 



4. Holectypus depressus. Interambulacral plate midway between apex and 



ambitus (No. 17 from the ocular). - X 5. 



5. Discoides cylindricus. "Negative secondary " in plan and section, xc. 30. 



6. Conulus albogalerus. "Encircled pit " in plan and section, x c. 30. 



7. ,, ,, Diagram of articulation of "spine" with pit. 



x c. 30. 



8. ,, ,, Plates midway between apex and ambitus 



(amb. III). x 5. 



In figs. 1, 2, 4, and 8 the several types of plate-ornament are in their correct 

 positions and proportions, but are represented by somewhat conventionalized 

 patterns. The outlines of the figures are based on tracings from photographs 

 as well as on actual measured drawings so as to ensure accuracy. 



