114 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



marking the center of the resulting spiral, and the mouth lying above the outer- 

 most coil (see fig. 21, p. 69). 



The articulation by which the first post-radial ossicle is joined to the radial varies 

 greatly in the different groups (see figs. 9-11, p. 65, 31, p. 71, and 431, 432, p. 349). 

 The dorsal (outer) portion is occupied by a large, more or less semicircular or ellip- 

 soid depression, the Dorsal ligament fossa which is bounded ventrally (anteriorly) by 

 a strong Transverse ridge upon which as a fulcrum the motion of the articulation is 

 accommodated; this ridge is usually undifferentiatcd, but hi one family it bears at 

 either end small triangular excavations known as Supplementary ligament fossae ; just 

 within the center of this transverse ridge is a deep pit, ending blindly, known as 

 the ligament pit ; just ventral (distal) to the center of the transverse ridge is a 

 canal which passes directly into the radial; this canal lodges the axial nerve cord 

 of the dorsal nervous system, and is called the Central canal ; it is sometimes, but 

 not always, surrounded by a raised rim; lying on either side of the central canal 

 are two shallow, usually triangular, but sometimes trapezoidal or even nearly 

 oblong or square, depressions with their bases lying along the transverse ridge 

 and their apices directed inward, the Interarticular ligament fossae; beyond these 

 are the deeper fossae, broadly rounded to narrowly linear, which accommodate the 

 muscles and are therefore called Muscular fossae ; these are separated in the mid- 

 line either by a narrow ridge, the Intermuscular ridge, or by a groove, the Inter- 

 muscular groove; and their inner distal corners are rounded off so as to form a 

 more or less deep Intermuscular notch. 



Within the radial pentagon, or the circlet formed by the radials in situ, there 

 is, in the oligophreate comatulids, a more or less dense secondary deposit of cal- 

 careous matter forming what is known as the Central plug (see fig. 11, p. 65). 



The centrodorsal is more or less excavated internally so as to accommodate the 

 chambered organ and accessory structures; the size of this cavity is variable; it is 

 very large in the macrophreate species, so that in some cases the centrodorsal is 

 reduced to a mere shell, but it is small in the oligophreate species (see figs. 

 267-273, p. 259, oligophreate species, 286-291, p. 262, macrophreate species). 



IDENTIFICATION OF RECENT COMATULIDS. 



While the keys given for the determination of the genera and species of coma- 

 tulids are ample for rapid and correct identification, as is the case with other groups 

 a certain amount of familiarity with the animals is essential in order that the differ- 

 ential characters given in the keys may be appreciated in their true relative value ; 

 much confusion may, however, be avoided if certain lines of procedure be followed 

 which, though as nearly as possible followed in the keys, are worthy of special 

 emphasis. 



The first structures to be examined in the determination of an unknown coma- 

 tulid are the arms; if these do not divide at all, and the cirri are irregularly arranged 

 on a discoidal or low hemispherical centrodorsal, the specimen belongs either to the 

 Pentametrocrinidse (5 or 10 arms) (figs. 113, 114, p. 181, 115-118, p. 183, 119, 

 p. 185, 120, p. 187, 121, p. 189, and 122, p. 191) or to the Zygometridae (5 arms) 

 (figs. S3, p. 136, and 184, p. 235); if the cirri are in 10 columns on a long conical 



