NOTE ON TETRAMEROUS SPECIMEN OF ECHINUS. 211 



normal genitals 1 and 5, ocular I. being entirely absent. 

 The absent ambulacrum is therefore I., the left one of 

 the three which normally form the trivium. 



Explanation of Plate XVII. 



Fig. 1. Diagrammatic figure of the peristome and 

 ambulacra, showing the exact positions of the 

 tegumentary gills, peristomial tube-feet and the 

 pair of tube-feet representing those of the absent 

 ambulacrum. 



Fig. 2. One of the pair of tube-feet marked tf in fig. 1, 

 magnified. 



Fig. 3. The ossicle os in fig. 2, viewed from above to 

 show the perforation through which the radial 

 water vessel opened into the tube-foot. 



Fig. 4. The "Lantern of Aristotle," viewed from the 

 side, to show the abnormal arrangement of the 

 two pairs of adductor muscles of the teeth. 



Fig. 5. Diagrammatic figure of the apical system of plates 

 X 3. The ocular plates are indicated by Koman 

 numerals, the genital plates by Arabic numerals. 

 The dotted outline indicates the position in the 

 system of the absent ocular plate. 



List of Keference Letters. 



am., ambulacra; ami., adductor muscles of the teeth; 

 au., auricula; m., mouth; mad., madreporite ; ped., 

 pedicellaria ; ptf., peristomial tube-feet; sp., spines; t., 

 test; tf'., abnormal tube-feet. 



