REPORT ON THE CRINOIDEA. 273 



from side to side, as is well seen in PI. LXVIII. fig. 1 ; but it is sometimes very much 

 restricted and not readily distinguishable from one of the larger ambulacra (PI. LXIV. 

 fig. 2). In a few exceptional cases the mouth is practically central, just as it is in 

 Antedon, though in other individuals of the same species it is nearly marginal (PI. LXII. 

 figs. 2, 4). 



In the endocyclic Crinoids the position of the mouth on the ventral side corresponds 

 very closely with that of the centre of radiation on the dorsal side ; though it is some- 

 times a little in front of the centre of the disk, as is well seen in Atelecrinus (PI. VI. 

 figs. 4, 6). But in any case the interambulacral area of the disk in which the anal tube 

 is situated corresponds precisely with an interradius of the skeleton, and the ambulacrum 

 opposite to it passes directly on to the joints of the corresponding ray, whether it is 

 undivided as in Eudiocrinus (PI. VI. fig. 2), or forks as in Atelecrinus (PI. VI. figs. 4, 6) 

 and Antedon (PL IX. fig. 2 ; PL XL. fig. 2 ; PL XLVII. fig. 2). In the latter genus the 

 displacement of the mouth, if it is not quite central, is always in the direction of this 

 anterior ambulacrum, and its position may therefore be described as radial. 



This is also true of a great many forms of Actinometra (Fig. 6, a). Thus for example 

 in the disk of an abnormal individual of Actinometra jimbriata, represented on PL LXII. 

 fig. 2, a median vertical plane would be radial in front of the nearly central mouth, and 

 interradial behind, where it would cut the anal tube. The same is the case in the more 

 typical specimen which is shown in fig. 4 of the same plate. In each alike there is a 

 short but wide anterior ambulacrum, which forks twice and so sends a branch to each of 

 the four arms on the anterior ray. Four more grooves leave the peristome in each 

 individual. But whereas in the one (fig. 2) each primary groove supplies all the arms 

 of one ray, just as in Antedon (PL IX. fig. 2), this is not the case in the other (fig. 4). 

 For each of the two antero -lateral grooves supplies but two arms of the corresponding- 

 ray ; and the two remaining arms receive their ambulacra as offshoots of the two primary 

 grooves, which supply the two postero-lateral rays, and together form a sort of horse- 

 shoe enclosing the anal interradius. This is much larger than in the endocyclic forms 

 (PL LXII. fig. 4), as the two hinder ambulacra curve outwards from one another towards 

 the margin of the disk, and so greatly reduce the size of the remaining interambulacral 

 areas. The anal tube is at the centre of the disk, and a vertical plane, cutting mouth 

 and anus, would pass along the short ambulacrum in front of the mouth, which is there- 

 fore radial in position, just as in Eudiocrinus and Antedon (PL VI. fig. 1 ; PL XL. 

 fig. 2). This condition may be traced, though less clearly, in the disk of Actinometra 

 elongata (PL LVII. fig. 3). 



On the other hand, there are a great many forms of Actinometra, as shown in the 

 diagram (Fig. 6, b), which have a distinctly interradial mouth. Two grooves start from the 

 sides of the peristome, instead of one from its anterior border (Fig. 6, a), and supply the 

 oral arms of the two corresponding rays (a 2 , Bj) ; while the ambulacra of their aboral arms 



(ZOOL. CHALL. EXP. — PART LX. 1887.) OoO 35 



