RELATIONS OF AMBULACRAL AND INTERAMBULACRAL AREAS. 233 



diate space ; therefore the greater importance, it seems, should be attached 

 to ambulacral areas and their variations. Ambulacral areas originate 

 in the young as 2 simple plates in all known Echini excepting Melo- 

 nitea and Lepidesthes, where they appear to originate as 4 plates,* and 

 also perhaps, excepting PhoUdocidaris. The addition of more columns 

 of plates, as in Oligoporus, or the production of compound plates, as in 

 Strongylocentrotus, takes place during the later stages in growth of the 

 corona, as shown by Mr Alexander Agassiz (1), Professor Loven (27), and 

 in the present paper. Therefore two columns of plates may be ac- 

 cepted as the usual characteristic of the whole class, which finds its rep- 

 resentative in the majority of adults, in nearly all young, and in the 

 adult of the simplest and oldest known type, Bothriocidaris. 



Interambulacral plates, as stated, are considered of secondary impor- 

 tance in the development of the corona, but they yield most important 

 facts, and especially in the Palseechinoidea are useful in separating and 

 tracing the systematic relationships of species and genera. Interambu- 

 lacral areas in all echinoids, as formulated by Professor Loven (27) and 

 as shown in several genera in this paper, originate ventrally in a single 

 plate. This single plate is shown in the young, and in the adult where 

 no ventral resorption of the corona has taken place.f Only one genus 

 is known which has a single column of interambulacral plates in the 

 adult, and that one is Bothriocidaris. This Lower Silurian genus there- 

 fore assumes especial importance, for the development of both ambu- 

 lacra and interambulacra and the geological position of the type all point 

 to it as representing the simplest known and, within the limitations of 

 the group, perhaps the simplest conceivable echinoid. 



The figures 3, 4, 5, page 234, show the relations of Bothriocidaris as a type 

 to Goniocidaris, which represents one great group of Echini characterized 

 by 2 columns of ambulacral and interambulacral plates. For representa- 

 tives of the other group of Echini, characterized by 2 or more columns 

 of ambulacral and 3 or more columns of interambulacral plates, refer- 

 ence should be made to the figures of Melonites, Oligo^iorus, Archseocidaris, 

 Lepidechinus and other genera, illustrated in figure 1, page 164; figure 1, 

 page 191, and accompanying plates. Attention is also directed to the 

 systematic classification table facing page 242. The initial plate V of 

 Bothriocidaris, figure 4, does not have an apex dorsally, as in all other 

 Echini, but this is obviously the result of the dynamic conditions, for Both- 

 riocidaris does not have two plates in the second row like all other known 



*Four plates are always found at the ventral border of Melonites, but some resorption of the 

 corona has taken place, and it is possible that the 4 plates would be found to pass into two, as in 

 Oligoporus, if young specimens could be obtained, as discussed on page 142. 



t Except in Arachnoides, as discussed on page 230, where 4 initial interambulacral plates have 

 disappeared in the adult from intercoronal resorption. 



