238 



MINERALS AND GEOLOGY 



FIG. 151. 



Simplified dissection of a crinoidal calyx, 

 viewed from below. 



usually regarded as the mouth, and, in living forms, an exceritric 



anal opening; but in most fossil 

 genera only one opening is pres- 

 ent. This is situated centrally 

 or sub-centrally, and is frequent- 

 ly placed at the summit of a 

 so-called "proboscis" or ele- 

 vated, tubular portion of the 

 calyx roof. The arms are in 

 some cases short and simple ; in 

 others, long and dichotomously 

 branched. As a rule, they are 

 free or separate, but sometimes 

 they are more or less united. 

 Commonly, also, they are pro- 

 vided with attached pinnulse, among which in living forms genera- 

 tion-products are developed. The plates which protect these arms 

 externally, form either a single regular series, as in a, figure 152; or 

 a single alternating series as at b ; or a double interlocking series as 

 at c. The same arm, however, sometimes presents two of these 

 conditions. The plates which compose the stem, 

 are circular or pentagonal (more rarely tetra- 

 gonal) in form, and usually shew a radiately- 

 striated surface on their planes of junction. 

 They are either of one diameter throughout the stem, or of alternating 

 diameters ; and they have always a central perforation, round, penta- 

 gonal, five-rayed, or rarely quadrate, in shape. Occasionally, addi- 

 tional orifices are also present. 



The Crinoids are usually sub-divided by palaeontologists into two 

 leading groups, named respectively, Tesselata and Articulata. The 

 Tesselata are distinguished essentially by the calyx-roof being closed 

 in with calcareous plates ; and by the calyx-plates, generally, being 

 comparatively thin and but loosely attached to each other : whilst 

 in the Articulata the roof has merely a coriaceous covering (strength- 

 ened in some forms by small plates or tubercles), and the calyx-plates, 

 as a rule, are comparatively thick, and in a measure locked together. 

 All the Tesselata are extinct ; and their remains (with the exception 

 of two Ci etaceous genera) are exclusively confined to Palaeozoic strata. 



