376 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



triangular i)rocessos of the rosette which are directed toward the sutures between 

 the five radials. 



The external faces of the radials bear complicated articular facets (figs. 9, 10, 

 p. 65, 431, 432, p. 349, and 439, 440, p. 351) to which arc joined the proximal ends 

 of the fii-st primibrachs, the first ossicles of the postradial series. These articular 

 facets may uiclmo at an angle of 45° to the dorsoventral axis of the animal, and 

 to the ventral s-urface of the centrodoreal, and thus be trapezoidal in shape, or 

 even nearly triangular, or they may bo parallel to the former, making an angle 

 of 90° with the latter, and thus be practically oblong. In most cases an inter- 

 mediate condition is found, and the general statement may be made that the 

 Macrophreata tend to approach the former extreme, the Oligophreata, especially 

 the more higlily specialized species, the latter. 



The articular facets are divided into one unpaired and four paired fossic (figs. 

 9, 10, p. 65), in a single genus Pontiometra (fig. 432, p. 349), a third additional pair 

 of fossffi being added, making a total of sLx. The doi-sal portion is occu])ied by the 

 large dorsal ligament fossa lodging the dorsal ligament, the function of which is to 

 antagonize the muscles; this extends as far as the transverse ridge, which stretches 

 transversely across the joint face and serves as the fulcrum upon which the motion 

 at the articulation is accommodated; just beyond the transverse ridge, one on 

 either side of the central canal lodging the dorsal nerve cords, lie the more or less 

 triangular interarticular ligament fossag, and beyond these, separated interiorly 

 either by a septum or a groove which reaches almost or quite to the central canal, 

 the muscular fossje, typically large and distally rounded, though often more or 

 less reduced and sometunes narrowly crescentic or Ihiear; they appear to be entirely 

 absent m the genus Pontiometra. 



The articular facet of the radials represents what is known as the straight 

 muscular articulation, the type of articulation from which all the brachial unions 

 are derived, as will be later explained. 



The distal bordera of the muscular fossse form the run of the funnel-shaped 

 central cavity of the radial pentagon, wluch extends downward to the rosette. 

 In the Macrophreata this cavity is usually comparatively small, but free from 

 calcareous deposit, while in the Oligophreata it is commonly much more extensive, 

 though more or less, often entirely, filled up by a loose deposit of calcareous matter 

 forming the central plug previously described upon which the visceral. mass rests. 



P. H. Carpenter noticed important differences m the composition of the radial 

 articular facets in such species as he was able to dissect, but he did not consider 

 them as offering available criteria for systematic work. From a somewhat more 

 extended study I have been led to the conclusion that the charactci-s presented 

 by the articular facets, and re])eatod witji progressively diminishing individuality 

 at all the muscular articulations throughout the postradial series, are of the highest 

 possible value in the delimitation of genera and higher groups, though scarcely 

 plastic enough, as a rule, to servo for the differentiation of species. 



I was led to pay particular attention to the systematic significance of the 

 radial articular facets from the fact that in the fossil comatulids the radial pentagon 

 together with the centrodoi-sal is commonly the only portion of the animal pre- 



