254 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



have been; specimens of species of Comastciidse or Tlialassomelridse, as well as of 

 Ilimeromolridffi or Colobomctridse, may be recovered from a mass of laval or coral 

 detritus wliicli has Ijccn tiirn<"d over and over in the dredge, and vet have practicall}^ 

 all the cirri intact. This is the more remarkable in the Thalassometridai, in which 

 family the sometimes enormously long cirri are often very slender. But in the 

 Macrojihreata the cirri are deciduous and, besides, very brittle, so that it is very 

 diflicult in many cases to recover any of them at all. This is the more unfortunate, 

 as the presence of the smaller apical cirri is such anomalous genera as PsatJiyro- 

 metra, Zenometra, Atupocrinus and Atclecrinus would give us a valuable clue to 

 their systematic affinities. 



There is a great diiference in the facility with which cirri are lost in different 

 genera, aiid this is always correlated with a corresponding facility of fracture in 

 the brachial syzygies. As a general rule the genera in which there is the most 

 resorption of the dorsal pole and the most proportionate increase in the thickness 

 of the widls of the centrodorsal have the most tenacious cirri; but this is to be 

 expected, since these genera, by these very characters, show the greatest approach 

 to the Oligophreata. Large species are less likely to lose their cirri easily than 

 smaller ones in the same genera, and in the same species large specimens are usually 

 more nearly perfect than smaller ones; but here again the large species or the large 

 specimens lake on certain oligophreate characters. The very small species, again, 

 are less liable to lose the ciiTi than the others on account of the immunity conferred 

 by their size. 



Of all the macrophreate comatulids the species belonging to the subfamily 

 Antcdoninai are the least liable to loss of cirri, with the species of Peroraetrinse a 

 more or less close second. The species of Bathymetrinse usually have at least some 

 of the cirri present, jilthough they are quite unknown in one of the species of Bathy- 

 metra. In the species of Ileliometrmse cirri are rarely founil m place; so far as I 

 have seen, when taken under ordinary conditions, not more than one in five or six 

 hundred specimens of the species of Solanometra, Heliometra, or of Ilaihrometra have 

 any cirri at all, and I have never seen a single specimen of any species of any one of the 

 three genera with the cirri perfect, although I have examined probably at least 50,000 ; 

 Promacliocrinus agrees with Ildiometra in this respect, as would be expected, but 

 the <'irri of Isometra and of TricJiometra appear to be somewhat more tenacious, 

 though the cirri of several species of the latter genus are as yet unknown. This 

 apparent tenacity may, however, be tlue m part to the fact that these genera com- 

 monly inhabit softer bottom. The Thysanometrinse are, as a whole, like the Ileho- 

 metrina?, though none of them retain the cirri so well as Trichometra; the cirri of 

 one of the species of Iridometra are unlaiown. In the Zenometrina^, Atelecrinidaj and 

 Pentametrocrinidaj specimens retaining even the basal segments of the cirri are 

 very rare, so that we are quite ignorant of their structure in half of all the known 

 species, including three entire genera. Of the seven genera, in only two, contauiing 

 two spetics each, are the cirri adequately xuiderstood. 



It is a fortunate circumstance that in two of these three groups with very 

 deciduous cirri the c'cntrodorsal is of the highest systematic value, presenting much 

 more important characters than the cirii. 



