RECONSTRUCTION OF ARCH^OCIDARIS WORTHENI. 219 



44. In this figure the areas are lettered as in the original figure 43. The 

 ventral row of plates in area / is restored completel}'', as it does not show 

 in the specimen. On account of the relative position of the plates I think 

 that this row would have the same arrangement as in areas E and 6r, as 

 discussed (page 216) , and have so restored it. In the restoration the buccal 

 pyramids are brought to an acuminate instead of truncated distal tip, 

 and the ambulacral and interambulacral plates are carried up on to the 

 peristome farther than they were seen in the specimen. Otherwise this 

 restoration differs from the facts as shown in the specimen only by being 

 made regular in outline, thus doing away with local distortion caused by 

 the processes of fossilization. 



It has been shown in several genera that in the development of the inter- 

 ambulacrum there is a single plate at the ventral termination in the young, 

 or in both the young and adult of some species. From this as a beginning 

 during progressive growth, there is an addition of new columns of plates 

 until the full number characteristic of the adult is attained.* It is further 

 shown that this initial plate is ordinarily resorbed. together with more or 

 less succeeding plates, by the encroachment of the peristome. Such being 

 the case, it is interesting to inquire into what was the structure of Archxo- 

 cidaris before resorption took place and how the four columns character- 

 istic of the adult interambulacrum developed. In plate 8, figure 45, an 

 endeavor is made to reconstruct these first plates, doing so by the laws 

 of growth as established in other genera in the present papers. Around 

 the mouth we must suppose that there existed a row of 10 ambulacral 

 plates, for such is found in the development of Cidaris (figure 3, page 234) 

 and Strongylocentrotus (plate 3, figure 8) and is known in the adult of the 

 oldest known echinoid, Bothriocldaris (figure 4, page 234). Whether this 

 first row of plates in Archseo cidaris was succeeded by another row of 10 

 ambulacral plates, as in Bothriocldaris^ as shown by Schmidt (37), or 

 whether it had an accelerated development, as in Cidaris (figure cited) 

 and skipped the second row, we have no means of telling. I prefer to 

 leave it as one row, as that is in accordance with the condition of most 

 genera observed. Succeeding the first row of ambulacrals, we suppose a 

 second row, in which there are two ambulacrals in each area, and, in ad- 

 dition, a single interambulacral plate, 1', exists as the beginning of that 

 part in each of the 5 areas.f In the succeeding rows as added, the am- 

 bulacra continue as two columns of plates, a, b, as in the adult. In the 

 third row there are in the interambulacra 2 plates in each area corre- 

 sponding to the condition existent in all genera above Bothriocidaris. 



* See studies of Melonites, Oligoporux, Pholido cidaris, Lepidocidaris, Lepidechinus, Cidaris, Strongy- 

 locentrotus, and Bothriocidaris, all illustrated by figures in the plates or text. 



t Compare this with plate 3, figures 8-11 ; plate 6, figure 27 ; plate 7, figure 42, plate 9, figure 54, and 

 figures 1, 3 and 4, pages 164 and 234. 



XXVII— Bull. Geol. Soc. Am., Vol. 7, 1895. 



