LINTHIA AUSTRALIS. 605 



Linthia australis 



! Desoria australis Gray. 1S51, Ann. Mag. N. H., VII. p. 132. 

 I Linthia australis A. Ac, 1872, Rev. Ech., Ft. I. p. 138. 



PI. XIX a . f.7-9 ; PL XXI ». /. 5 - 7. 



Test thin ; outline from above elliptical, angular. Anterior groove de- 

 scending to the ambitus ; posterior extremity vertically truncated. Apical 

 system anterior, about one third the length from the anterior edge ; vertex 

 central. The central part of the lateral interambulacral plates form a 

 series of irregular ridges, extending from the apical system to the ambitus. 

 These ridges are less marked in small specimens. The posterior interambu- 

 lacrum rises to the vertex from the apical system, extending as a more or 

 less angular keel, most distinct at the apical part, towards the anal system; 

 odd anterior ambulacrum broad, diverging to the ambitus ; pores closely 

 crowded near the apex, becoming quite distant at the ambitus, extending to 

 the actinostome ; intervening space covered by large distant miliaries. The 

 lateral ambulacra are sunken; the anterior at right angles to the odd ambu- 

 lacrum, elongate, curved anteriorly at the extremity, reaching almost to the 

 ambitus. The posterior petals somewhat shorter, diverging near the ex- 

 tremity ; poriferous zones of lateral ambulacra broad, as broad as the median 

 bare interporiferous spaces ; pores of zones large, round, joined by indistinct 

 grooves, well separated vertically ; apical system small ; madreporic body 

 elongate ; four genital openings, equidistant, or posterior sometimes di- 

 verging. Peripetalous fasciole narrow, angular, with a re-entering angle in 

 each median interambulacral space ; the deepest angle in the lateral poste- 

 rior interambulacra. It crosses the anterior groove near the ambitus, forming 

 an acute anode across the ambulacrum. The lateral fasciole starts above the 

 extremity of the anterior lateral petal, sloping towards the ambitus; near 

 the posterior edge it forms a sudden curve, and passes under the anal system 

 at the ambitus, a considerable distance below it. The actinal surface is 

 slightly concave ; the actinostome near the anterior edge small, broad, with 

 distinct phyllodes, and bare posterior ambulacral avenues. The actinal plas- 

 tron is broad, triangular, slightly re-entering at the posterior extremity. 



The anal system is high, pointed, narrow, covered with an outer row of 

 large rectangular plates, the largest at the lower part of the system, and 

 with smaller inner concentric rows, gradually decreasing in size towards the 

 central anal opening. 



The tuberculation of the abactinal surface is small, uniform, with the 



