586 HEMIASTEE AUSTEALIS. 



defined anal fasciole, together with the subanal fasciole, as in Brissopsis, 

 forms the subgenus Rhynobrissus ; the typical Hemiaster includes only 

 small, short Echini, with a depressed test, truncated posteriorly, with- 

 out a well-defined actinal plastron, and more or less sunken ambulacra, 

 while the high conical species resembling Micraster, with bare vertical 

 median interamhulacral and ambulacral sutures, with an indistinct anal 

 fasciole and an imperfect actinal plastron, form the subgenus Faoriua. 

 It seems almost useless to attempt a better definition of these subgenera 

 with the limited material at our command, anil when additional specimens 

 are discovered we may find better reasons than we now have for adopting 

 or discarding the subdivisions of the geuus Hemiaster, and the association 

 of the species as here proposed. 



Hemiaster australis 



! Tripylus australis Phil., 1845, Wieg. Archiv., p. 347. 

 I Hemiaster australis A. A.;., \±:i. Rev. Ech., I'i. I. p. 132. 



Pi XXI. J. .:. 



The outline of the test of this species varies considerably seen from above. 

 In some specimens it is exactly like the outline of II. cavernosus. — angular, 

 pointed posteriorly : while in other specimens in which the test is. perhaps, 

 slightly mure depressed, the outline is more regularly elliptical, nearly as 

 broad as long. These specimens Professor Loven is inclined to consider as 

 distinct species, though the material at our command seems hardly sufficient 

 for a determination of the question. There is also a slight difference in the 

 proportion of the anterior and posterior ambulacra! petals, and the peri- 

 petalous fasciole is usually broader than in the angular specimens ; hut in 

 both these features we find considerable difference in the specimens 1 have 

 been able to examine, which leads me to suppose that additional material 

 would show a range of variation great enough to include, as characteristic 

 of one species, all the differences mentioned above. 



The apical system is more central than in II. cavernosus ; the depth of 

 the anterior ambulacral groove is the same, but the lateral ambulacra are 

 only slightly sunken. The pores of the poriferous zones vertically distant 

 the poriferous zones as broad as the median space ; ambulacra slightly petal- 

 oid, straight ; the posterior petals sometimes very slightly arched outward 

 at the extremity. The pores of the odd anterior ambulacrum are especially 

 distinct as far as the peripetalous fasciole. This, with the exception of the 



