656 MESSRS. RITCHIE AND MciNTOSH ON [June 16, 



the ambulacral areas took place about the time that ambu- 

 lacrum Y ceased to grow. The derangement was in most cases, 

 however, only a tempoi'ary one, for in all the areas, save No. Y, 

 the elasticity of the organism appears to have overcome the 

 functional disorder, and after the formation of a few unusxial 

 plates, the normal growth was resumed. JSTevei-theless, a slight 

 indication of after-effect is afforded by differences in the size of 

 the tubercles, for a cursory glance shows that they are smaller 

 above the affected zone than below it. This variation, however, 

 is somewhat discounted by the fact that even in normal speci- 

 mens a similar, though less marked, difference in size exists 

 between the tubercles above and below the ambitus *. 



The disappearance of the two rows of ambulacral plates from 

 area V rendered necessary considerable modifications in the test, 

 and this regulation was cai'i-ied out mainly in two ways. The 

 plates of the adjoining interambulacral areas increased a little 

 in length and considerably in depth, and closing in around the 

 truncated area came together in the mid-line, where they were 

 united by a zigzag suture. As a direct result of the increase of 

 the plates, the sutures in the neighbourhood became distorted. 

 The increase in the size of the interambulacral plates, however, 

 was not sufficient to make up for the loss of a double row of 

 ambvdacral plates measuring over 15 mm. across, hence another 

 modification became necessary in order that the space between 

 the adjoining areas on each side might be spanned. This was 

 brought about by the plates passing directly across the space 

 instead of building a material- wasting globular dome, the result 

 being evident in the flattened portion of the test which lies 

 between the truncated ambulacrum and the apical disc. 



The increase in the depth (that is, direction of short axis) of 

 the plates, and the flattening of the surface of the shell have 

 together had the effect of pushing the incomplete ambulacrum 

 further from the apical area, so that it has come to form the 

 centre of a distinct bulge in the outline of the test, while it has 

 also given rise to an abnormally flattened area on the oral surface. 



It was perhaps at this period of disturbance that the terminal 

 tentacle disappeared and that fresh deposits of calcareous matter 

 began to close up the unoccupied ocular pore. 



Relation to Previously Described Oases op a 

 Similar Character. 



Mr. W. Bateson has brought together the cases of abnormality 

 ill the major symmetries of Echinoids recorded prior to 1894 f. 

 The remaining records up to 1902 are mentioned by Hamann J. 



* F. Jeffrey Bell : ' Catalogue of British Ecliiuoderms in the Brit. Mus. (Nat. 

 Hist.),' p. 152. London, 1892. 



t Bateson, W., 'Materials for the Study of Variation,' pp. 441 et sen., London, 

 1894. 



X Hamann, Otto, I. c. p. 1293. 



