PART 5 A MONOGRAPH OF THE EXISTING CRENOIDS 45 



irregular and crowded rows about the margin, in some genera, as in Eumetra, Irido- 

 metra, Argyrometra and Andrometra the cirrus sockets become aligned in regular alter- 

 nating rows, usually three or four, but sometimes as many as six, and the centrodorsal 

 may acquire a more or less conical form; indeed in Andrometra indica it bears a strong 

 resemblance to that of the species of Hathrometra. This increasing regularity in the 

 arrangement of the cirrus sockets is correlated with increasing slenderness and delicacy 

 in these organs and a proportionate decrease in the size of their sockets, while the 

 assumption of the conical form shows the same correlation and is accompanied by the 

 retention of small apical cirri. Both of these features are developed in the deeper 

 and more quiet regions, but the former is noticeable to a considerable extent in certain 

 of the small littoral species of Dorometra. 



The cirri in the various species of this subfamily are exceedingly variable. While 

 they afford, broadly speaking, the best and most reliable characters for the differen- 

 tiation of the species, the several kinds overlap so completely that they are quite useless 

 for purposes of generic definition; in Antedon alone practically all the types found in 

 all the other genera are represented. 



The length of the cirri varies from 4 to 27 mm. (40 mm. in Eumetra chamberlaini) 

 or from 10 to 33 (48) percent of the length of the arms; in the different species of 

 Antedon alone it varies from 10 to 28 percent of the arm length. In 34 of the species 

 where both measurements are included in the descriptions the percentage is usually 

 (in 21 species) from 16 to 25 percent, sometimes (in 9 species) between 10 and 15 per- 

 cent, more rarely (in 3 species) between 25 and 35 percent, and only in Eumetra cham- 

 berlaini nearly 50 percent. The average for all species is 20 to 21 percent, so that in 

 this group the cirri may be said to vary from a tenth to nearly half the arm length, 

 with an average of about a fifth and a range between a tenth and a quarter, longer 

 cirri being very exceptional. 



The number of the cirri varies from X-XX to L-XC, and is most commonly XXVI- 

 XXXV; in only a few species are there more than L. There appears to be no definite 

 relation between the number of the cirri and the size of the animal or the depth inhabited. 



The number of segments in the fully developed cirri varies from 10 to 33, with the 

 greatest frequency between 14 and 16, several species having 12 or 13; only half 

 as many species reach 17 as 16, and half as many 18 as 17; in only 5 species does the 

 maximum number exceed 18, and in only 4 does the minimum for fulty developed 

 peripheral cirri reach 22 or more. 



The form of the cirri varies from short and stout, strongly recurved in the distal 

 half, with the longer proximal segments about twice as long as broad and the short 

 distal segments, which are strongly compressed laterally and may appear about twice 

 as broad as the proximal in lateral view, about as long as wide and with a prominent 

 opposing spine (as in Toxometra, Annametra, Mastigometra, and Antedon petasus, 

 A. dwbeni, A. bifidamoroccana, A. arabica, and A. incommoda) to elongate and slender, 

 almost straight, with all the segments except the basal greatly elongated, and tapering 

 to a sharp straight point distally with no opposing spine (as in Antedon longicirra 

 and approximately in Eumetra chamberlaini) ; but they usually fall towards the first 

 of these two extremes. 



Short and stout cirri are characteristic of the larger littoral species (in AnteJon, 

 Mastigometra, Annametra, and Toxomitra), while delicate cirri with much elongated 



