212 ECHINODERMATA— PELMATOZOA phylum iv 



rigU the primary brachials, and to the left an anal tube or sac ; the first plate of 

 this, corresponding to the anal x, may he entirely above the level of the radials, or, as 

 usually, may slightly indent their upper corners at the posterior interradial suture ; 

 but never fully enters the dorsal cup. Arms non-pinnulafe, uniserial, dichotomous or 

 heterotomous. Eadiäl facets usually wide and straight. Tegmen not well known. 

 Ordovician and Silurian. 



Heterocrinus Hall (Stenocrinus W. and Sp.). Crown subcylindrical, calyx 

 small. Three radials transversely, more or less equally bisected, or Com- 

 pound ; these being, in addition to the right posterior, the right and left 

 anterior, or sometimes the anterior in place of the latter. Anal tube delicate 

 and straight; first tube-plate resting on the Shoulders of both posterior 

 radials, but not further entering the cup. Arms irregularly dichotomous, 

 somewhat divergent. Stern pentagonal, quinquepartite, with interradial 

 sutures. Ordovician ; North America. 



Ohiocrinus W. and Sp. Calyx and stem as in Heterocrinus. Arms hetero- 

 tomous, having ten main branches not in close contact, and somewhat sinuous, 

 with ramules which usually brauch again. Ventral sac large, and usually 

 convoluted. Ordovician ; North America. 



Eäenocrinus S. A. Miller {Heterocrinus W. and Sp., non Hall). Calyx about 

 as in the preceding. Arms heterotomous, with ten main branches, straight, 

 rather closely abutting, composed of a continuous series of syzygies of two 

 plates each, the epizygals giving off ramules. Stem round, tripartite. 

 Ordovician ; North America. 



locrinus Hall. Only one radial Compound, the right posterior, the lower 

 part of which is of about the same size as the other radials, which are all large. 

 The superradial is short, resembling an axillary brachial, supporting on its 

 right Shoulder an arm and on the left a series of plates f orming the arm-like 

 dorsal ridge of a strong anal tube or sac, of complicated structure ; first tube 

 plate entirely above the level of radials, and not entering the dorsal cup at all. 

 Arms dichotomous, branching frequently. Stem pentagonal, quinquepartite, 

 with interradial sutures, the pentameres radially disposed. Ordovician ; 

 North America. 



Herpetocrinus Salter (Ophiocrinus Charlesw.; Myelodactylus and (?) Brachio- 

 crinus Hall). A specialised form with crown of the locrinoid type, habitually 

 enclosed by the coiled stem, whose structure is modified accordingly. The 

 crown is rarely seen, being beut backward, and usually closely enveloped by 

 the stem, which is then coiled around it in the opposite direction. One ray is 

 dwarfed, either not branching or entirely aborted. The right posterior radial 

 alone is Compound, the superradial supporting the series of tube-plates entirely 

 above the level of the radials, as in locrinus. Anal tube long and narrow, 

 composed of a series of heavy plates resembling brachials dorsally, with 

 perisome on the other side. Arms more or less irregularly dichotomous. The 

 stem could be tightly coiled, or uncoiled exposing the crown ; but the latter 

 condition is rarely found in the fossils. Resulting from this the stem has lost 

 its cylindrical form, being more or less concave at one side, with its columnals 

 crescentric in section, and bearing on the horns of the crescents two longitudinal 

 rows of strong cirri. The remarkable resemblance of the coiled cirriferous 

 stem to a pinnulate arm has misled many students, for the crown is usually 

 concealed. Silurian ; North America and Europe. 



