chap, xvii.] ECHINOIDEA. 443 



anterior ambulacrum was missing, whether it be the anterior ambu- 

 lacrum, or the left anterior, or the left posterior that is wanting, or 

 even if all 4 new ambulacra correspond with all 5 of the normal.] 



9. Amblypneustes sp. (S. Australia): four specimens, each with four ambulacra 

 [no description or statement as to symmetry]. Haacke, Zool. Am., 1885, p. 505. 

 (See No. 687.) 



(2) Partial or total disappearance of a definite ambulacrum or 



interambidacrum. 

 0. Echinus melo, having only four complete ambulacral areas (Fig. 

 137). The specimen is not spherical, for the apical system is warped 

 over in one direction and the oral pole is pulled in an opposite direction, 

 while the shell is much higher in the region of the apical system than 

 it is at the opposite side. There are only four ocular plates, which are 

 subequal, the madreporic plate and the plate opposite to it being some- 

 what larger than the other two. The genital plates are also four. 

 Only four ambulacral areas leave the apical system, and at that point 

 they are almost symmetrically disposed. Lower down however a 

 triangular series of plates bearing ambulacral pores is intercalated 

 between the plates of one of the interambulacral systems which it 

 divides into two. This intercalated series is of course the representa- 

 tive of the ambulacral area which is wanting at the apex of the shell. 

 The five ambulacra are nearly symmetrically disposed round the oral 

 surface just as the four ambulacra are round the apical system. This 

 transition from a tetramerous to a pentamerous symmetry is effected 

 by complementary changes in the amount of divergence of the rays as 

 they pass down the shell. Examination shews that the ambulacrum 

 which is thus partially absent is the right posterior. Philippi, Arch, f 

 Naturg., in. p. 241, Plate. 



Amblypneustes formosus : a 4-rayed specimen having a 

 somewhat asymmetrical test. One of the interambulacral regions is 

 abnormally wide, and at about 9 plates down the side of the test in this 

 region a wedge-shaped piece composed of several partially distinct 

 plates bearing 7 pairs of ambulacral pores. This fragment doubtless 

 represents the deficient ambulacral area. The apical system consists of 

 10 plates. The two genital plates of the abnormal area are reduced in 

 size, and the ocular plate between them is abnormally large. Consider- 

 ing the madreporic plate as indicating the right anterior interambula- 

 crum, it appears that it is the left anterior ambulacrum which is thus 

 deficient. The heisrht of the shell at the abnormal side is less than at 

 the other. Bell, E Jeffrey, Jour. Linn. Soc., xv. p. 126, Plate. 



In each of the foregoing the missing ambulacrum is actually at some 

 place represented by plates of ambulacral character, and the shape of 

 the test is greatly changed in correlation with the partial disappearance 

 of the radius. The following cases differ, in that in them one ambula- 

 crum is wholly wanting in the affected radius, and the interambulacra 

 are contiguous with each other. Curiously enough in two of these 

 specimens the symmetry is changed little or not at all. The cases in 

 Hemiaster were all Algerian fossils '. 



1 Besides those here given in the text, Gauthier in the same place describes 

 an interesting case of symmetrical reduction in the two posterior ambulacra of 

 Hemiaster africanus. 



