I 



FBOM THE AUSTRALIAN TEEIIARIE3. 



419 



sutural edge of the ambnlacral plate, is a row of close and small 

 granules. The peristomial termination of each groove is in two 

 terminal ambnlacral plates, which are narrow, and each pair of 

 ambnlacral plates is separated from its neighbours, on either side, by 

 a single peristomial interradial plate. The second ambulacral pair of 

 plates are larger than the peristomial, and are sutured along the 

 median line of the interradia with the neighbouring pair of ambu- 

 lacral plates, there being no interradial plate intervening. This 

 discontinuity is as in Chjpeaster. The second plates are marked 

 with obliquely arranged granules, and are pierced by numerous 

 small pores, so that two triangular areas are thus marked, the base 

 of each being in contact along the median line of the ambulacrum, 

 one side being along the aboral suture of the plate, and the third 

 side being along a line drawn from the peristomial end of the median 

 suture of the plate across to the aboral and interradial angle of the 

 plate. A corresponding ornamentation occurs on the next and some 

 other pairs of plates, and is comparable with the peculiar appearance 

 shown by A. Agassiz in his drawing of the actinal surface of Chjpe- 

 aster Mavehellii, A. Ag. (" Echini of the Blake Exped.," Mem. Mus. 

 Comp. Zool. Harvard, vol. x. Xo. 1, 1S33, pis. xv. b, xv. c). 



There are very few pores to be seen in the groove abactinally, 

 and the ambulacral plates beyond the petaliferous part are crowded 

 with minute pores close to the groove, but not far from it. 



The position of the periproct is inframarginal and rarely at the 

 margin, and there is slight swelling of the test (not so much as in 

 Laube's figure) actinally and abactinally along the path of the 

 intestine in relation to the posterior interradium. 



The jaws are not seen in any of the specimens, but it appears 

 that the low perignathic ridge-process is placed, as in Chjpeaster, 

 upon the edges of the peristomial ambulacral plates. 



The coronal plates, actinally, are well marked near the margin of 

 the test, and the petaloid parts of the ambulacra resemble those of 

 Cli/peaster ; but the interradia are greatly diminished in breadth 

 near the apical system. 



The apical system is small and the madreporite is central and 

 button-shaped, the four genital pores beiDg immediately external 

 to it. There is no posterior genital opening. 



Two young specimens show that the notching of the margin and 

 the distinctness of the abactinal grooves are matters of growth ; and 

 it is quite pardonable for any naturalist who is aware of the great 

 diversity of shape assumed by Clypeastroids in their youth to place 

 the young forms out of the genus or subgenus Monostychia. It is 

 evident that while there are some structural resemblances to 

 Arachnoides in the actinal part of the test, the principal structural 

 characters of physiological importance are all Clypeastroid. 



There are. not sufficient data to separate the species from the 

 very flat Clypeasters, and I propose to retain Monostychia as a sub- 

 genus of Chjpeaster. 



