424 PELMATOZOA. 



roughly speaking, that the calyx is disproportionately large and 

 massively constructed as compared with the arms, interradials 

 being usually present and often united with the radials in such 

 a way as to form a portion of the calyx; while the anal inter- 

 radius is specially developed, thus rendering the cup unsymmet- 

 rical. Moreover, the mouth and food-grooves are generally con- 

 cealed from view by the development of a more or less definite 

 "vault" above the proper ventral surface of the calyx. On the 

 other hand, the Neocrinoidea are all Secondary, Tertiary, or Recent, 

 and are roughly distinguished from the Palaeocrinoids by the com- 

 paratively small size of the usually symmetrical calyx and the pro- 

 portionately large development of the arms. The interradials, if 

 present, are rarely incorporated into the calyx, and with one excep- 

 tion (Thaumatocrinus) an "anal" interradius cannot be recognised. 

 The higher radial plates are more or less movably articulated, and 

 do not enter into the composition of the calyx. Lastly, the ventral 

 surface of the visceral mass is not covered by a plated dome, but 

 the mouth and ambulacral grooves are exposed to view. Speak- 

 ing generally, the division Palceocrinoidea may be regarded as cor- 

 responding with Miiller's division of the Tessei/ata, while the section 

 Neocrinoidea corresponds with the Articulata and Costata of the 

 same author. 



As regards the distribution of the Crinoids in space, the order is 

 represented by comparatively few forms in recent seas, and these 

 have mostly a very local range. All the existing forms belong to the 

 division of the Neocrinoids, and the majority of them are referable 

 to the free-living family of the Comatulidce, of which there are not far 

 from two hundred known forms, belonging to six genera (Antedon, 

 Actinometra, Atekcrinus, Eudiocrinus, Promachocrinus, and Thauma- 

 tocrinus). On the other hand, there are only about forty known 

 living types of the " Pedunculate " Crinoids, belonging to some half- 

 a-dozen genera (Pentacrinus, Phizocrinus, Bathycrinus, Hyocrinus, 

 Metacrinus, and Ho/opus). The Co?natuIidce have a very wide range 

 in space, being found in almost all seas, but they are essentially 

 inhabitants of shallow water. Many of the living stalked Crinoids, 

 on the other hand, such as Bathycrinus and Hyocrinus, are only found 

 at great depths in the sea. They do not range, however, below 

 2500 fathoms, while one species of Antedon occurs at a depth of 

 2900 fathoms. 



As regards their distribution in time, the entire group of the 

 Palaeocrinoids is restricted to the Palaeozoic period, and may be said 

 to have attained its maximum development in rocks as ancient as 

 the Silurian. Throughout all Palaeozoic time the Crinoids are the 

 predominant types of the Echinoderms, and many of the Ordovician, 

 Silurian, Devonian, and Carboniferous limestones are so extensively 



