ANATOMY OF THE TEST OF DISCOIDEA CYLINDRICA. 



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 Bibs." — 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, aud their height is about a millimetre 

 near the girdle, and 3 millimetres at the opposite extremity. 



There are two ribs to an interradial area, one on each side of 

 the median line of suture of the iuterradium ; and each one is 

 placed along the middle of each row of interradial 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 interradial 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 Periproct. — This is small in relation to the size of the test, 

 and is in a very slight concavity in the odd iuterradium, being 

 nearer to the margin of the test than to the peristome. It is 

 elongated in the direction of the ant ero -posterior axis of the test, 

 and varies in sha^De 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 betw^een them. The larger tubercles are m sunken 

 scrobicules which are shallow ; and there is decided crenulation 

 and perforation. 



*Zittel, Handb. d. Pal. Bd. i. 1876-80, p. 514, fig. ,373. The ten ribs are 

 shown and part of tlie perignathic girdle. 



