a. h. clark : the crinoidr of thk indian ocean. 59 



17. — Key to the Genera of Thalasso metrics. 

 ai P| shorter and smaller than P,, but similar to it. 



b| cirrus sockets close together, irregularly distributed, but tending to arrange 

 themselves in fifteen columns, or in three columns in each radial area ; 12 — ;!0 

 arms. 



ptilometra, p. 189. 



b'' cirrus sockets not crowded, arranged in ten columns, two in each radial area, 

 c' cirri comparatively stout distally, with the ventral surface smooth ; 10—20 

 arms. 



A.STEROMETRA, p. 190. 



c* cirri slender distally, the proximal segments bearing long ventral spines: 

 15 — 30 arms. 



pterometra, p. 189. 

 a* P| larger and longer than P.^. 

 b' ten arms, 

 c' calyx and arm bases spinous. 



thalassometra, p. 195. 

 0* calyx and arm bases smooth. 



d' calyx and arm bases rounded, not carinate ; cirri very stout. 



orotalometra, p. 209. 

 d* IBr series stiongly carinate. 



e» Pj only slightly larger than P.^ ; arms strongly carinate throughout. 



STENOMETRA, p. 209. 



e* P, much larger than P.^ ; arms rounded , not carinate. 



STIREMETRA, p. 210. 



b^ more than ten arms, 



c' calyx and arm bases very spinous ; P, very much larger than P.j. 



THALASSOMETRA, p. 195. 



0* calyx and arm bases not spinous ; P, not greatly larger than P, : division 

 series all 2. 

 d' division series and arms strongly carinate throughout. 



STENOMETRA, p. 209. 



d* division series and arms with no trace of, or with only a very faintly 

 indicated, carination. 



e' less than 30 cirrus .segments; genital pinnules short ; lateral flatten- 

 ing of rays not marked. 



PARAMETRA, p. 213. 



e* more than 40 cirrus segments ; genital pinimles moderately long : layf^ 

 and division .series (when present) sharply flattened laterally. 



COSMIOMETRA, p. 214. 



