Herbert L. Hawkins — Studies on the JSchinoidea, etc. 197 



inconspicuous ornament consists of no less than five different types 

 (see PI. XIII, Figs. 1 and 2). 



The primary tubercles are set in shallow areolae, and are arranged 

 serially in a definite pattern (see Hawkins, Geol. Mag., 1911, p. 448). 

 They are few in number and cover but a small proportion of the 

 surface of the plate. The secondary tubercles are very small, but 

 are fairly numerous. They are areolate, but some of them seem not 

 to possess mamelons. At times they form rough rings around the 

 primary areolae, and occasionally take on an irregularly linear 

 arrangement in a transverse or radiating direction. The greater 

 number of them seem to be quite fortuitous in position, and they 

 vary considerably in diameter. Still more irregular, both in 

 distribution and size, are the miliary granules, which are present 

 in about the same numbers as the secondaries. 



In addition to these three normal types of ornament, destined for 

 the support of primary and secondary radioles and pedicellariae, are 

 two more of quite different appearance. One of these series of 

 "tubercles" superficially resembles that of the secondary tubercles. 

 The structures are of about the same size, but differ in the 

 comparative scarcity of the special series, and in the character of 

 their areolae. They are either flush with the surface of the test 

 (though readily visible owing to their extreme smoothness), or more 

 commonly, slightly raised above the surrounding level. In the 

 centres of these smooth circular areas are placed minute and in- 

 conspicuous mamelons, which seem to be always imperforate. The 

 mamelons rise from pits of very little greater diameter than them- 

 selves, and do not project above the level of their "areolae" (see 

 PL XIII, Pig. 5). 



The other aberrant series consists of tubercles which may be of 

 any size from that of a secondary to almost that of a primary, 

 having small mamelons on bosses set in areolae which are deeply, 

 but not entirely r sunk below the plate surface. When a test has 

 been stained with a granular substance, this type of ornament 

 becomes very obvious through the accumulation of colouring matter 

 in the hollows. Although many of the sunken tubercles are situated 

 without apparent order, some few seem to be fairly constant in 

 position on the serial plates of a column. 



The former series of peculiar "tubercles" will be more fully 

 described in the section of the paper dealing with Conulus, in which 

 genus similar structures are much more strongly developed, but 

 the latter series (apparently absent from Conulus) requires more 

 precise analysis. 



Except that the mamelons of the "sunken tubercles" seem to be 

 usually, if not always, imperforate, the chief difference between 

 these structures and the normal primary tubercles lies in the 

 character of their areolae. There seems always to be some sort of 

 a boss on which the mamelons are placed, though there is hardly any 

 indication of its base. The floor of the areola is inclined, to make 

 an angle with the plane of the test-surface, and the inclination is 

 always adoral and inwards. In most cases the adapical border of 

 the areola is sensibly raised above the surrounding level. Thus the 



