H. L. Hawkins — Tuberculation of Holectypoida. 443 



(adapical) in position, in an Echitiocardium the most strongly- 

 developed are situated almost entirely on the inferior (adoral) surface. 



These contrasts, in common with most of the others, between 

 the two great groups of Echinoids, are, to a considerable extent, 

 bridged over by the Holectypoida. In view of the very scanty 

 evidence available for a study of the actual acanthology of the group, 

 the proportions of the tubercles have to be regarded as furnishing 

 an index of the character of the radioles. The investigation may 

 be taken along two lines ; the characters of the individual tubercles, 

 and their changes in proportion and structure in different parts 

 of the same test, and the changes in the order of distribution of 

 the tubercles on the plates, which appear on a comparison of different 

 species and genera. 



The importance of the arrangement of the primary tubercles, as 

 a guide to genetic sequence, has not received the recognition it 

 deserves. Most authors, when figuring new species of Echinoids, 

 give an enlarged drawing of one or more interambulacral plates 

 to show this feature, but the meaning of it is rarely emphasized 

 in the text, and often, owing to the not infrequent irregularities 

 of growth in individual specimens, the figure becomes positively 

 misleading. Almost the only occasion of which I am aware, when 

 this character was exhaustively studied for systematic purposes, 

 was in a paper by Saemann and Dollfuss in 1861 (Bui. Soc. geol. 

 Erance, ser. 11, vol. xix), where a distinction was diagnosed between 

 Pygaster gresslyi and P. laganoides (P. morrisi being united to the 

 latter species) on the evidence of the tuberculation. It is very 

 doubtful whether the arrangement of the tubercles in the Spatangids, 

 or even in the ' Cassidulids ; , could ever be used for the purpose of 

 accurate diagnoses of species or even genera, its extreme complexity 

 and consequent tendency to irregularity rendering a study of the 

 interambulacra in these forms somewhat bewildering. 



In the more complex Regular Echinoids, and in all the Holectypoids, 

 the primary tubercles occur in regular vertical series in the inter- 

 ambulacra. These series appear with marked regularity until the 

 ambitus is reached, after which they disappear in a corresponding- 

 order. There is always one row, approximately median in its position 

 in the half-interradius, which persists from the apex to the peristome, 

 and is commonly more prominent than the others. The other series 

 of tubercles may be described as occupying the interradial or ad/radial 

 tracts, according as they occur on one side or the other of this 

 median series. The irregularities of development, which occur 

 naturally, become more frequent as the complexity of the tubercula- 

 tion becomes more marked. Eor the most part the diagrams here 

 given are generalizations, which omit the numerous individual 

 peculiarities. 



In the Holectypoida the tuberculation shows a progressive com- 

 plexity of arrangement when it is traced from the Jurassic to the 

 Cretaceous representatives. As Saemann and Dollfuss showed in 

 the paper already referred to, the value of this feature as a specific 

 index depends very largely upon the age of the individual — a question 

 often difficult to solve in the case of fossil forms. Ear more evidence 



