392 Herbert L. Hawkins — Studies on the Echinoidea. 



marginals, would then be left as a flat, sloping plate of large 

 size. This is certainly its general appearance in a specimen, but 

 with careful lighting it may be found that the ridge plate is not 

 flat but has a marked central concavity and several small irregularities 

 on the exposed face. Its free (distal) margin is never, so far as 

 I have determined, so straight as in the figure, and there is always 

 a decided change in the direction of that margin when the processes 

 are reached. But unless this large ridge represents simply the 

 primordial interambulacral plate (which is almost inconceivable) it 

 must necessarily be compound, and be crossed by sutures vertically 

 or transversely or both. Duncan & Sladen expressed surprise at 

 the apparent absence of sutures, but seem to have been satisfied that 

 they were not present in the specimen from which their diagram was 

 drawn. Their remarks (I.e., p. 237) imply that they were prepared 

 to believe that sutures might exist in the ridge in some specimens 

 and not in others. Such variability would be contrary to all 

 experience in Echinoid structure, and one can only believe that their 

 specimen was unsuitably preserved or cleaned for the display of the 

 sutures. 



To sum up : the lateral lappets of the ridge in Duncan & Sladen's 

 diagram are in reality processes, and there is every reason to believe 

 that the lack of sutures in the broad median plate is due to incomplete 

 observation. 



(b) Loven's Interpretation. (PL XXV, Fig. 2.) 



The beautiful figure published by Loven in 1888 was copied by 

 him in 1892, with the addition of another in which no sutures were 

 shown. The twice published figure (here copied) was taken from 

 the girdle of a depressed form of D. cylindricus, while the other 

 represented a modification of the processes which Loven thought to 

 be restricted to the elevated, cylindrical form. Loven suggested 

 that the striking diversities between the superficial aspects of the 

 two girdles might imply a specific difference between the two forms. 

 In 1909 I showed that the "forma elatior" type of girdle could be 

 found in specimens of the "forma vulgaris" (or " depressa"), and 

 subsequent observations have confirmed this. The difference between 

 the two types consists in the greater prominence of the processes in 

 the "forma elatior" and in their inappreciable elevation beyond the 

 apex of the ridge in the depressed form. This would appear to be 

 a modification due solely to the size (i.e. age) of the specimen. The 

 occurrence of the two kinds of girdle in the same form in no way 

 vitiates the possibility of there being two distinct varieties of the 

 species ; on the contrary, it makes it the more probable that the two 

 forms are definitely distinct. While the largest specimens of 

 D. cylindrieus that I have seen are all of the "forma elatior ", I have 

 examined numerous specimens of the "forma vulgaris" which were 

 much larger than many decidedly cylindrical examples. 



To turn to an analysis of the figure here copied : The outline of 

 the girdle is for all practical purposes similar to that shown in 

 Duncan & Sladen's figure. But the sutures at the bases of the 

 processes are represented as curved, somewhat irregular lines, while 



