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 i)lates arc 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 (proxmial) 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 etlge of the disk 



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



figs. 25-28, p. 69). 



Marginal furrows arc oidy foimd in the families Comastcriila3 and Uinta- 



crinida;. 

 Marginal mouth. — A mouth is said to be margmal when it is situated ujx)!! the 



margui of the disk, in the center of a horseslioe-shaped margmal ambidacral 



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

 Median column. — Of cirrus sockets; tlie mitlradial columns in each radial area 



(sec 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 hno (see figs. 435, and 445a, p. 351, and p. 374). 

 Midradial gap. — The bare midrailial 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 opcnmg of the digostiA'e tube, situated at the focus of the disk 



ambulacra; it occu])ies the center of the disk m all comatuUils except those 



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



ComastcridiB (see figs. 15-19, p. 67, and pp. 110-111). 

 MuUibrachiate. — IlaAong more than 10 arms; that is, possessmg IIBr series; 



this term is not apphetl to the sjiecies of Pramachocrinus wliich have 20 arms, 



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

 Multiplicative regeneration. — See Regeneration A4. 

 Muscle 2>lates. — (1) The Muscular fossx. 



(2) The articulating surface of a muscular articulation. 

 Muscular articulations. — See Articulations. 

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



of a muscidar articulation, servmg for the attachment of the muscles (see 



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



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



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

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

 Nonmuscular articulations. — See Articulations B. 

 Non^tentaculiferous arms. — See Grooveless arms. 



