94 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Loose suture. A union between two contiguous calcareous plates formed of amor- 

 phous connective tissue, by which the plates are but loosely joined together 

 (see Suture) . 



Lumen. The interior cavity of a more or less tubular structure. 



M. 



Marginal cirri. The cirri developed along the inferior (proximal) margin of the 



centrodorsal (see figs. 81, p. 134, and 85, p. 139, and pp. 294-295). 

 Marginal furrow. An ambulacial furrow which runs along the edge of the disk 



in a horseshoe-shaped course, the month being in the center of the furrow (sec 



figs. 25-28, p. 69). 



Marginal furrows are only found in the families Comasteridse and Uinta- 



crinidse. 

 Marginal mouth. A mouth is said to be marginal when it is situated upon the 



margin of the disk, in the center of a horseshoe-shaped marginal ambulacral 



furrow (see figs. 25-28, p. 69). 

 Median column. Of cirrus sockets; the midradial columns in each radial area 



(see figs. 198, p. 237, 200, p. 239, and 208-214, p. 241, and pp. 244-247). 

 Middle pinnules. See Genital pinnules. 

 Midradial furrows . Furrows on the inner or ventral faces of the radials occupying 



the median line (see figs. 435, and 445a, p. 351, and p. 374). 

 Midradial gap. The bare midradial area, bounded on either side by a lateral 



column of cirrus sockets, seen in certain types of centrodorsals (see fig. 196, p. 



237). 



Midradial intermuscular furrow. See Intermuscular furrow. 

 Mouth. The anterior opening of the digestive tube, situated at the focus of the disk 



ambulacra; it occupies the center of the disk in all comatulids except those 



belonging to the genus Uintacrinus, and most of those belonging to the family 



Comasteridse (see figs. 1519, p. 67, and pp. 110-111). 

 Multibrachiate. Having more than 10 arms; that is, possessing IIBr series; 



this term is not applied to the species of Promachocrinus which have 20 arms, 



arising from 10 radials, each post-radial series dividing once. 

 Multiplicative regeneration. See Regeneration A4. 

 Muscle plates. (1) The Muscular fossse. 



(2) The articulating surface of a muscular articulation. 

 Muscular articulations. See Articulations. 

 Muscular fossae. The most distal (ventral) pair of fossas on the articulating surface 



of a muscular articulation, serving for the attachment of the muscles (see 



figs. 9-11, p. 65, 31, 32, p. 71, and 431, p. 349, and p. 114). 



N. 



Naked disk. A disk upon which no calcareous deposits are visible under ordinary 



examination (see figs. 15-17, p. 67). 



Nodals. In the pentacrinites, the columnals which bear cirri (see fig. 127, p. 197). 

 Nonmuscular articulations. See Articulations B. 

 Non-tentaculiferous arms. See Grooveless arms. 



