TERMINOLOGY 23 



The mode of union between the plates of a crinoid is either by suture, or articulation. 

 The first may be by close suture, loose suture, or anchylosis or fusion. A close suture is an 

 immovable union in which the apposed surfaces are flat, and may be smooth or striated, and 

 the plates are united by short fibres of amorphous tissue : e. g., between the calyx plates of the 

 Camerata. In a loose suture the faces are more or less concave or excavated, lodging bundles 

 of ligament, with contact areas of smooth facets or of interlocking crenulations, thus allowing 

 a certain amount of motion between the plates : e. g., between the calyx plates of the Flexibilia, 

 and to a lesser degree between the stem ossicles of most Paleozoic crinoids. 



Anchylosis is a modified close suture in which the lines of union have been obliterated by 

 subsequent deposit of stereom, or fused. Syzygy is a special kind of close union usually by 

 flat striated faces, sometimes called a non-muscular articulation, found only in arms and stem 

 interposed between two of the ordinary joints and by which two ossicles are rigidly united 

 into one compound ossicle. 



Articulation is a movable connection between plates of the radial series and between 

 columnals of some crinoids. It may be : ( 1 ) Upon undifferentiated joint-faces, by concavo- 

 convex surfaces, usually without transverse ridge : Cyathocrinidae, and senile condition in 

 some Recent crinoids. (2) With fossae, paired muscles and ligaments, but mostly without 

 transverse ridge: Flexibilia, with slight exceptions. (3) Complete muscular articulation, 

 with highly developed facets, fossae, transverse ridge, and paired muscles and ligaments : 

 Poteriocrinidae sensu Wachsmuth and Springer; Mesozoic and Recent crinoids with a few 

 exceptions ; and in the flexible free arms of the crinoids generally. For details of other articu- 

 lations more especially applicable to the Recent crinoids, see Clark, Monograph of the Exist- 

 ing Crinoids, p. 66. 



The orientation is based upon the natural position of the crinoid, with the arms upper- 

 most, and viewing the specimen from the anal side. The anal interradius will then be pos- 

 terior, the radius opposite to it anterior, while the right and left sides will correspond with the 

 right and left of the observer. In illustrations the dorsal view is drawn with the anal 

 interradius up, and the ventral view with the anal side down. Right and left remain the same 

 in both cases. Next to the anterior ray are the right and left anterior or antero-lateral rays, 

 or more conveniently called lateral rays ; and adjoining the anal interradius the right and left 

 posterior rays. Corresponding appellations are applied to the interradial spaces, which con- 

 sist of the two anterior, the two postero-lateral, or simply lateral, and the posterior or anal 

 interradius. 



Dorsal. The surface opposite to that which includes the mouth and ambulacra : aboral. 

 Under normal conditions it is the lower external surface of the crown, as opposed to the sur- 

 face next to the viscera. 



Ventral. The surface upon which is situated the mouth and ambulacra : adoral. Upper- 

 most under normal conditions, and when speaking of plates the surface or direction toward 

 the viscera. 



Proximal and distal are reckoned from the plane separating stem from crown, so that the 

 infrabasals and the top columnal are the proximal elements of crown and stem, respectively. 

 In the crown, the outer surface of a plate is the dorsal side, the lower edge the proximal face, 

 its upper edge the distal face, and the faces at the sides are the lateral faces. In the stem, the 

 upper face is the proximal, and the lower the distal. Proximate, a term applied to the upper- 

 most columnal. 



