238 



ECHINODEEMATA— PELMATOZOA 



PHYLUM IV 



first brachial, never on the second. If the arms are undivided the pinnules are 

 usually excessively long and reach to the arm tips ; hut the length of the pinnules 

 decreases in proportion to the number of arm divisions ; pinnulars sometimes tending 

 to fuse into a solid piece. Column as in the Apiocrinidae, but never with a proximal 

 enlargement. 



Plicatocrinus von Miinster (Fig. 341). Radials comparatively thin, their 

 articular facets crescent-shaped ; the outer faces longitudinally folded into 

 a median ridge. Arms ten, dividing on the first brachial ; composed of 

 wedge-shaped ossicles united by perforate muscular articulation. Pinnules 

 composed of a single piece, except the proximal ones, which consist of three 

 pieces ; they are angular or keel-shaped along the dorsal side, and deeply 

 A _ . o _ :i> furrowed on the ventral. Teg- 



men unknown. Upper Jura; a 

 rare form, found in the Fran- 

 conian and Swabian Alb. 



Hyocrinus Wyv. Thomson 

 (Fig. 342). Three basals ; five 

 arms, bearing extremely long 

 pinnules which reach to the 

 arm tips ; brachials united in 

 syzygial groups of three. Teg- 

 men composed of five large 

 orals, surrounded by heavily 

 plated perisome. Recent ; 

 Antarctic Seas. 



Gephyrocrimis Koehler and 

 Bather. Similar to Hyocrinus, 

 but brachials united in syzy- 

 gial groups of two ; proximal 

 portion of column pentagonal. 

 Recent ; Canaries and Madeira. 

 Thcdassocrinus A. H. Clark. 

 Similar to Gephyrocrinus, but 

 proximal portion of column 

 hexagonal. Recent ; Philip- 

 pines. 



Ptilocrinus A. H. Clark. 

 Five arms, unbranched ; each brachial, except the most proximal, bears a 

 pinnule ; syzygies very infrequent ; pinnules not especially long. Recent ; 

 Antarctic Seas and west coast of America to British Columbia. 



Calamocrinus A. Ag. Five brachials ; the arms branch several times. 

 Recent ; Galapagos Islands and Central America. 



Pig. 341. 



Plicatocrinus hexagonus Miinst. Upper 

 Jura ; Streitberg, Franconia. a, Calyx with 

 radials and undivided base ; &, c, Dorsal and 

 lateral aspects of same (slightly enlarged) ; 

 d-f. First brachial, seen from the inside, out- 

 side, and from below respectively. 



am 



-OJ; 



Fig. 342. 



Hyocrinus hethelUanus Wyv. Thomson. Recent; Atlantic 

 Ocean. A, Individual twice the natural size. B, Tegmen several 

 times enlarged ; am, Ambulacral furrows of the arms ; c, Dorsal 

 canals ; an, Anus ; m. Mouth ; o, Orals (after Wyville Thomson) ; 

 *', Covering plates of ambulacral grooves. 



Family 6. Saccocomidae d'Orbigny. 



Calyx small, hemispherical, non-pedunculate, composed almost exclusively of 

 five radials, which are very thin, elevated into prominent ridges along the median 

 line, and enclose an extremely small hasal plate. Arms 5x2, slender, widely 

 separated, and giving off alternately toivards the extremities simple incurving 

 branches. Arm-plates cylindrical; each side of the ambulacral furrow lined with 



