62 Triassic Echinoderms of Bakony. 
adradial; two poriferous; and one median perradial. A. Gras, Desor, DuNcAN, 
and others have called this last tract «the interporiferous zone or area», a term which is liable to 
confusion with the interporal space between the two pores of a pair. A less cumbrous 
term for the poriferous tract is pore-field. To use the term «zone» for these meridional 
tracts, as was originally proposed by A. Gras «pour éviter toute confusion» (1848, p. 298), 
seems a curious distortion of its original meaning. C. DEsmouuins (1835, p. 135) called 
the perradial tract «la portion interambulacraire», since in his terminology the interradial 
areas were «anambulacraire». (Text-fig. 8.) 
A main Tubercle with its immediate surroundings consists of the following parts, 
beginning at the centre (Text-figs. 8, 9.), The mamelon, which may be perforate by a circular 
or elliptic foramen or imperforate, with its dome hemispherical or depressed 
hemispherical, with circumference circular or slightly distorted from a circle so as 
to be meridionally elongate or transversely elongate, i. e. parallel to the 
equator, The elongation of the mamelon is often at right angles to that of its foramen. 
The neck of the mamelon, which comes immediately below the dome may be straight, 
i, e. with sides parallel to the axis of the tubercle, or slightly undercut, The neck is 
foramen dome of mamelon 
Ak _ undercut neck 
e- excavate platform 
- 
straight neck - 
_- parapet 
flush platform ----._ 4 
scrobicular circle 
‘ 
Text-fig. 9. Section of a main tubercle. The right and left halves are represented as being different. 
borne by the boss, and usually rests on a definite platform cut at right angles to the 
axis of the tubercle. This platform may be flush, or excavate so as to appear surrounded 
by a parapet. The parapet or, in its absence, the edge of the platform, may be crene- 
late or plain (briefly expressed by «tubercle crenelate or non-crenelate»), The outer wall 
of the platform or of the parapet may be vertical for a short distance, or the boss may 
at once begin to slope away with a straight, concave, or convex slope, which may be 
continuous until it dies away in the scrobicule, or may be interrupted by a ledge, 
defined in life by the attachment of the internal radiolar muscles, and forming a kind of 
subsidiary platform — the basal terrace (A. Tornguist, N. Jahrb. f, Mineral., 1896, 
Il, p. 85). The scrobicule is usually sunk below the level of the extrascrobicular surface, 
and thus forms a moat or fossa around the boss. The outer edge or vallum of this moat 
is the scrobicular circle. This is usually surmounted by a definite series of 
scrobicular tubercles constituting ascrobicularring, Even when the scrobicule 
is flush with the extrascrobicular surface, and when consequently the scrobicular circle 
is not in evidence, the limits of the scrobicule are generally marked by the scrobicular 
ring. If this also is undeveloped, then the scrobicule is distinguished from the extrascrobicular 
surface by the general ornament of tubercles or granules on the latter. This discrimination 
between the scrobicular circle and the ring of scrobicular tubercles may conflict with 
the usage of some eminent authorities, but appears in accord with the language of the first 
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