184 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



THALASSOMETRA OMISSA (Koehler) 



Antedon omissa KOEHLER, Re'sultats des campagnes scientifiques accomplies sur son yacht par Albert 

 1 r Prince souverain de Monaco, fasc. 34, 1909, p. 268 (description; Princesse- Alice station 1713, 

 1904), pi. 32, fig. 10. MORTENSEN, Handbook of the echinoderms of the British Isles, 1927, 

 p. 25. 



Thalassometra omissa A. H. CLARK, Proc. U. S. Nat. Mus., vol. 40, 1911, p. 7 (occurs on the north- 

 western coast of Africa) , p. 37 (synonymy; locality); Unstalked crinoids of the Siboga-Exped., 

 1918, p. 166 (in key; range), p. 171 (synonymy); The Danish Ingolf-Exped., vol. 4, No. 5, 

 Crinoidea, 1923, p. 40 (range), p. 57 (in key). -MORTENSEN, Handbook of the echinoderms of 

 the British Isles, 1927, p. 25. 



Diagnostic features. The cirri are arranged in 15 crowded columns; the elements 

 of the division series and lower brachials are smooth dorsally except for a few minute 

 spines along the proximal and distal edges- and the brachials have the distal border 

 thickened, forming a transverse crest that bears a few very fine spines. There are 10 

 arms, and the cirri are about 17 mm. long with 21 segments. 



Description. The centrodorsal is hemispherical, with the cirrus sockets apparently 

 arranged in 15 columns. 



The cirri are XX +, 21, about 17 mm. long. The two first segments are very short, 

 the third is slightly longer, the fourth and fifth are much longer, and the sixth, which 

 is the longest, is about twice as long as broad. Beyond this point the length gradually 

 decreases to the twelfth which, with the segments following, is about as long as broad. 

 From the sixth segment onward a small spine appears toward the distal end which, 

 gradually moving toward the middle of the segments, continues to the end of the cirrus. 

 There are X principal cirri and a few other smaller ones that rise vertically between 

 the arms. 



The radials are partially visible. The IBri are almost twice as broad as long with 

 the distal border slightly incised by the IBr 2 (axillaries), of which the dorsal surface 

 is slightly convex, the distal angle is rather sharp, and the dorsal surface is depressed 

 toward the central portion in such a way that the articular border is somewhat pro- 

 jecting. The elements of the IBr series and the first brachials are slightly flattened 

 laterally. The diameter of the proximal portion of the animal is 4 mm. 



The 10 arms are all broken, the longest arm stump being only 30 mm. long. The 

 first two brachials are short, broader than long, and those following gradually elon- 

 gate, at the same time becoming narrower. The dorsal surface of the first two brachials 

 is depressed in such a way as to render the proximal and distal edges, especially the 

 latter, slightly projecting. The brachials following show the same central depression, 

 the distal edge projecting as far as the twelfth or fifteenth. Under a lens it is seen that 

 this raised edge is provided with very short spines difficult to make out that appear 

 from the seventh or eighth bracliial onward. 



Syzygies occur between brachials 3 + 4, again between brachials 11 + 12, or some- 

 times 12+13, and distally at intervals of from 4 to 6 muscular articulations. 



PI and P are large and well developed with about 12 segments of which the 

 first three or four are broader than long, those succeeding becoming elongated and 

 very slender. They are slightly carinate. Under the microscope these pinnules show 

 very fine spines, short and sparse, much less abundant than the spines in Th. echinata. 

 P 2 is very short, with only 6 segments. The pinnules following increase slightly in 

 length. On about the eighth there are 8 segments, and a little farther on, 9. The 



