378 Prof. S. Lovdn on the Structure of the Echinoidea. 



The great distinction between theCidaridae and the Latistellge 

 consists in this dissimilar movement of the ambulacra. Thus, 

 in the former, the ambulacrum breaks up at the peristome into 

 its elements, the primary plates, and these become converted 

 into free imbricated lamellse ; and this is in complete opposition 

 to the homologous movement which, in the Latistellge, con- 

 cludes with their union into large plates, which in the peri- 

 stome coalesce into a solid ring. And nevertheless the same 

 law prevails here most distinctly as in all other Echinoidea. 

 A careful examination shows that, of the oldest scales (which 

 immediately surround the mouth), those which belong to series 

 I. a, II. a. III. 5, IV. a, V. h are somewhat smaller than those 

 which constitute series I. &, II. &, III. «, IV. 5, V. a. If we 

 trace the rows upwards towards the corona, we find that the 

 former throughout cover the latter with their lateral margins ^ 

 and when we come to the corona we see that this position of 

 theirs is due to the fact that the plates of the former series 

 come later to the margin, and do not begin to separate until 

 after their fellows of the latter series are already completely 

 free and begin to arrange themselves in the imbricated rows 

 of the buccal membrane ; and this difference is to be recognized 

 throughout the whole ambulacrum. Of each pair of plates,, 

 that which belongs to the series I. a-V. h follows that of the 

 series I. 5-V. a; and of the newly formed pair close to the eye- 

 plate, the plate belonging to the former series is always less 

 developed than that of the latter series, or even still uncom- 

 menced. In a Cidaris hystrix of 28 millims. diameter the 

 number of plates from the mouth to the vertex is as follows : — 



in I. a 54 in I. ft 55 



„ II. « 54| „ II. 6 55i 



„ III. h 54 „ in. a 54i 



„ IV. a 66 „ IV. 6 66i 



„ V. & 56 „ V. « 56i 



If we compare with the Latistellge the different groups of 

 irregular Echinoidea, Echinoneus comes nearest to them in the 

 structui'c of the ambulacra. All the pores are double pores. 

 As in the Spatangidge and the Cassidulidse, the peristomial 

 plates of series I. a-V. h have two pores (that is to say, they 

 consist hypothetically of two early coalescent primary plates), 

 and those of the series I. J-V. a only of one ; and in the 

 former the lowest pore is marginal and broken, and frequently 

 has its upper tube obliterated, as in the Echinidge. But the 

 buccal membrane is without pore-plates. Although the row 

 of pores is simple, entire and half plates alternate with each 

 other in regular order, which does not continue the same from 

 tlie vertex to the peristome. The newly formed plates are 



