16 STRUCTURE OF THE CRINOIDE^. 



could gain access to the interior by that route. Accordingly a more 

 direct passage is provided. In a great many species which have no 

 calyciual grooves there is an aperture at the base of each arm in 

 which the groove of the arm terminates. I think that in such 

 species the ambulacral vessels, after descending from the extremity 

 of the arms to the bases of the arms, pass directly into the body 

 through these apertures. I have therefore, in Decade III., proposed 

 to call these the " ambulacral orijices.^^ 



The genera in which they are most conspicuous are Actinocrinus, 

 Kfiodocr'imis, and Platycrinus, but there is abundant evidence of their 

 presence in a great many other genera. The subject of these 

 orifices requires a good deal of investigation, as there appear to be 

 many important modifications upon which our Canadian specimens 

 throw no light. In fact all our Lower Silurian species are in such 

 a bad state of preservation, that many years must elapse before the 

 whole of their characters can be worked out. The principal varia- 

 tions in the position of these orifices exhibited by diflferent of the 

 Crinoids, are the following : — 



1. Crinoids with the ambulacra continued from the base of the 

 arms ovei- the ventral surface to the mouth — no ambulacral orifices 

 distinct from the mouth, — Comatula, Pentacr'mus, PalcBocrinus. 



2. Crinoids with the ambulacra continued from the bases of the 

 arms to a single ambulacral orifice in or near the centre — the mouth 

 on one side — Spharocrimis, Crotalocriiius, PalcEocrinus, Carahocrinus. 



3. Crinoids with an ambulacral orifice at the base of each arm, 

 the mouth either central or between the centre and one side. 



The following figures exhibit some examples of the variations 

 in the position of the mouth and ambulacral orifices in different 

 species : — 



Pig. 2. Fig. 3. rig- 4. 



Figure 2. View of the ventral surface of Sphcerocrinus gcometricus (Goldfuss sp.). 

 ?», the mouth. 

 " 3. Ventral surface of iiAorfomnws jjcrws. m, the mouth. 

 " 4. Crotalocrinus. (From Sir R. I. Murchison's Siluria, 1st edition, p. 219.) 



