CLASS III CRINOIDEA 215 



Arms long, giving off armlets alternately at intervals. Column round or pentagonal ; 

 in the latter case having its angles radially directed, and cirri which are interradial. 

 Lower Carboniferous. 



Belemnocrinus White (Missouricrinus S. A. Miller). The only genus, very 

 rare. Burlington Group ; Mississippian area, North America. 



Family 7. Dendrocrinidae Bather. 



Dicyclic. Structure of legmen not well known, probably composed chiefly of 

 undifferentiated plates, more or less extended into a tube or sac, sometimes resembling 

 an arm proximally, and usually with anal opening at the distal end. Arms uniserial, 

 either dichotomous and strictly non-pinnulate, or heterolomous with main rami bearing 

 lateral ramules tending to incipient pinnulation. Loose, irregidar interbrachials 

 occasionally present in lower part of interradius in some genera. Radianal in 

 primitive position in form of radial under the right posterior ray. Badial facets 

 wide or narrow; mode of union with proximal brachials not well known, but 

 probably by modified or imperfect muscular articulation. Infrabasals five. Stem 

 ■usually round, sometimes pentagonal and quinquepartite. Ordovician and Silurian. 



This assemblage of early genera may be considered as a sort of composite family, in which 

 are embraced a number of characters which later became fixed as valid family criteria. They 

 are all primitive in the position of the radianal, and therein differ from all later Fistulata. 

 The presence of interbracJiials irregularly in some genera, e.g. Gupulocrinus and Otfawacrinus, 

 which are foreign to the Inadunata, indicates a close relation to the Flexibilia ; and as 

 Springer has shown, there are good reasons for considering the first of these genera as very 

 close to the ancestral type of the two orders. 



Merocrinus Walcott. Arms dichotomous, branching. Eadial facets wide, 

 shallow, nearly straight. No anal plate in line with radials. Anal tube at 

 the base resembling an arm branching from the left side of the axillary right 

 posterior superradial. The genus might be considered as a dicyclic locrinus. 

 Ordovician ; North America and England. 



Cupulocrinus d'Orb. (Scyphocrinus Hall, now Zenker). Arms and radial 

 facets about as in the preceding. Large anal x in line with radials, truncate 

 above, supporting a large tapering anal tube, with a median row of large 

 plates dorsally, bordered by perisome, rising only about half the height of 

 the arms. Small irregular interbrachials often present in primary axils. 

 Ordovician ; Canada and Kentucky. 



Thenarocrinus Bather. Dorsal cup similar to the preceding, but radianal 

 slightly to left and touching infrabasals. Anal tube large and long, composed 

 of transversely folded plates, without median ridge or perisome. Silurian ; 

 England. 



Dendrocrinus Hall. Radial facets narrow and semicircular. Arms dicho- 

 tomous, branching many times, and very slender. Anal x in line with radials, 

 angular above. Anal tube wide, long, composed of hexagonal plates in 

 vertical parallel columns. Ordovician and Silurian ; North America. 



Oftaivacrinus W. R. Billings. Arms heterotomous, with ten main branches, 

 bearing lateral ramules, which may subdivide, or may approach the pinnulate 

 stage. Anal x in line with radials ; tube wide, rising the full height of the 

 arms. Radial facets wide and. nearly straight. Ordovician ; Canada. 



Gothocrinus Bather. Similar to preceding, but with radial facets narrow 

 and curved, and shorter ramules. Silurian ; Gotland. 



