308 BULLKTIN 82, UNITED STATES NATIONAL MUSEUM. 



muscle plates of the successive brachials, and the vascular structures which are 

 partly contained in it are continued from one brachial to another between the 

 two large muscle bundles which unite them. In the Oligophreata and in very 

 large species of the Macrophreata this arm groove merely lodges the lowest part 

 of the coeliac canal, while the genital cord, with the water vascular and blood 

 vascular tubes and the ambulacral epithelium are all situated above the arm 

 groove and separated from it by a variable amount of intervening perisome, so 

 that little more than half the vertical height of the arm is due to its dorsal 

 skeleton. In the majority of the Macrophreata, however, a great part, sometimes 

 even the whole, of the soft parts of the arm are lodged within the groove on the 

 upper surface of the skeleton, and there is no substantial ventral perisome in the 

 ordinary sense of the word, or it is reduced to a mere film, sometimes thinly 

 plated, which covers up the muscular bundles. 



The outline of the disk may be approximately circular or pentagonal (figs. 712, 

 p. 346, and 747-752, p. 349) , in which case the disk is said to be entire, or there may 

 be more or less deep reentrants in the interradial areas (figs. 713-724, p. 346), when 

 the disk is said to be incised. 



In one family, the Pentametrocrinidse, the disk is very strongly stellate (figs. 

 758, 759, p. 353, and 1158, pi. 25), due to the extraordinary extension over the 

 proximal portion of the arms. 



Incision of the disk reaches its extreme in the families Himerometridae (figs. 

 713-718, p. 346), Stephanometridse (fig. 726, p. 346), and Mariametridfe (figs. 719- 

 725, p. 346, and part 1, fig. 17, p. 67), in the genera Zygovietra (figs. 710, 711, p. 

 346, and 1155-1157, pi. 25) and Oatoptometra of the Zygometridae, and in the genera 

 Cenometra (part 1, fig. 16, p. 67), Cyllometra and Epimetra of the Colobometridse, 

 and is marked in the Thalassometrinre (figs. 743, 745, p. 349) and in the Chari- 

 tometridse (figs. 746, p. 349, and 1159-1161, pi. 26). 



The di.sk is most nearly circular in outline in the Calometridse (figs. 734-737. 

 p. 349, and part 1, fig. 19, p. 67), in many of the Antedoninse (figs. 750, 756, p. 349), 

 and in the genus Eudiocrimis (fig. 712, p. 346) of the Zygometridae, in which it is 

 almost globular in form, and it is very strongly rounded in the other subfamilies 

 of the Antedonidae (figs. 747-757, p. 349), in the Tropiometridae (figs. 729-733, 

 p. 346, and part 1, fig. 15, p. 67), and in the Ptilometrinse (figs. 738-742, p. 349). 



The interradial borders are slightly incised in the Comasteridae (figs. 685-709, 

 p. 341; 1144, 1147, 1148, pi. 23; 1149-1153, pi. 24; and 1154, pi. 25), the Colobo- 

 metridse (figs. 727, 728, p. 346), excepting the genera Cenometra, Cyllometra, and 

 Epimetra, and the Atelecrinida (figs. 1163, 1164, pi. 26). 



Thus while between the shape of the disk and the systematic position of the 

 comatulids there is but slight correlation, between it and the number of arms 

 there is a much more definite connection, for, broadly speaking, the larger the 

 number of arms the deeper are the reentrants between the five arm groups; but 

 it should be emphasized that even here the correspondence is only approximate, 

 and we find a notable exception in the Comasteridae. 



The ventral surface of the disk may be (1) quite naked (though calcareous 

 deposits are always present in the cutis) (figs. 748, 749, 751, 752. 75'1^757, p. 349), 



