188 Scientific Proceedings, Royal Dublin Society. 



(b) One form is a stellate, the second a sanidaster. 



3. Sub-family. SANIDASTEEINA. 



No other flesh spicules are present — Tribrachmm. 

 Trichite sheaves are present as well — Tethyopsis. 



(c) One form is a stellate, the second an amphiastrella. 



4. Sub-family. STRYPHNINA. 

 A single genus — Stryphnus. 



(d) One form is a stellate, the other a spined bacillus. 



5. Sub-family. PSAMMASTERINA. 



A single genus— Psammastra. 



Although this classification appears to be wholly based on the 

 flesh-spicule, it is not so in fact ; but it happens as a remarkable 

 coincidence that differences in the flesh-spicule are as a rule asso- 

 ciated with other and profounder differences in the organism : we 

 might easily have brought the latter more prominently forward in 

 this classification, but it would have involved more space than we 

 can here afford. 



Genus 1. Myriaster. — Sponge small; oscules distinguishable 

 from pores ; ectosome thin, mainly collenchymatous, excavated by 

 widely extending sub-dermal cavities, which are never restricted to 

 form chones. Flesh spicules, chiasters only. (The chiaster is a 

 small stellate, with an excessively minute centrum, hair-like rays 

 either abruptly truncated at the ends, or capitate ; usually few in 

 number. The typical forms, with few rays and capitate ends, may 

 be fancifully supposed to represent the Greek letter x, hence the 

 name chiaster). The mesoderm is a sarcenchyme, the flagellated 

 chambers small, usually about 0*02 mm. in diameter; they open 

 by short abiti into the excurrent tubes. Distribution chiefly in 

 Australian seas. 



Myriaster subtilis, sp. n. — Sponge small, lobate ; a few small 

 oscules. Spicules— (1) acerate, 1*35 to 1*5, by 0-032 mm. ; (2) fork ; 

 shaft, 1*2 by 0'04 mm. ; rays bifurcate ; primary rays, 0'042 ; 

 secondary, 016 mm. long; (3) anchor, shaft, l"16by 0012 mm. ; 



