ANATOMY OP THE TEST OF DISCOIHEA CYLINDEICA. 
59 
that the teeth did not project, but moved as in the Clypeastroids. 
It is possible that the dental apparatus was slender, as it is in 
Holectypus. 
The “ Internal RibsT - — These ten ridge-like projections on the 
interradia within the test are low and narrow, but wider at their 
bases than at the free edge. They reach from the outer and 
under part of the perignathic girdle to the inner surface of the 
test just above the ambitus, and their height is about a millimetre 
near the girdle, and 3 millimetres at the opposite extremity. 
There are two ribs to an iuterradial area, one on each side of 
the median line of suture of the interradium ; and each one is 
placed along the middle of each row of iuterradial plates. 
The ribs are not additional plates, but simply special upward 
growths of the middle portions of the upper surfaces of the inter- 
radial plates of the actinal part of the test. The sutures which 
are between consecutive iuterradial plates can be traced over 
the ribs. Finally, it must be noticed with regard to the growths 
of the inside of the test, that there is a low ridge along each 
median suture of the five ambulacra on the inside of the base 
of the test ; it commences at the ambulacral part of the 
girdle, and gradually diminishes towards the ambitus of the test 
within *. 
The Teriproct . — This is small in relation to the size of the test, 
and is in a very slight concavity in the odd interradium, being 
nearer to the margin of the test than to the peristome. It is 
elongated in the direction of the antero-posterior axis of the test, 
and varies in shape from the elliptical with rather narrow" ends to 
the ovoid with a considerable curvature posteriorly. It is bounded 
by four plates in one and five plates in the opposite zone ; and the 
sixth plate from the peristomial margin, not including the first 
single plate, forms the inner boundary in one zone, and the seventh 
plate in the other zone. 
The surface of the test, as has been remarked by nearly every 
observer, is remarkably punctate ; and the multitudes of minute 
depressions have a corresponding number of small, close, sharp 
granules between them. The larger tubercles are m sunken 
scrobicules w'hich are shallow" ; and there is decided crenulation 
and perforation. 
* Zittel, Handb. d. Pal. Bd. i. 1876-80, p. Oil, fig. 373. The ten ribs are 
shown and part of the perignathic girdle. 
