CHARACTERS OF BASICORONAL PLATES. 69 



Dorsally each interambulacrum comes in contact with a genital and two adjacent ocular 

 plates, except in Bolhriocidaris archaica (Plate 1, fig. 2) and in part in B. pahleni (Plate 1, fig. 6), 

 and in the posterior interambulacrum of many spatangoids (Micraster), where the interambula- 

 crum abuts against two ocular plates only (text-figs. 174, 175, p. 149). The interambulacrum 

 also comes against oculars only in the remarkable variations in Arbacia shown in Plate 4, figs. 

 11, 12. The new interambulacral plates in Echini are formed against the ocular plates on 

 either side of the area, never in the middle against the genital. 



The relation of the interambulacrum to the ambulacrum in development is of much interest, 

 and it seems may be expressed in terms of acceleration of development. In Bothriocidaris 

 (Plate 1, fig. 1) two continuous rows of ambulacral plates surround the mouth, forming the 

 peristome of that type, before reaching the initial plate of the interambulacrum in the third 

 aboral row or zone. This character in Bothriocidaris is discussed in detail under consideration 

 of that type. In young Goniocidaris (Plate 2, fig. 1) and in young Strongylocentrotus (Plate 3, 

 fig. 11), young Salenia (A. Agassiz, 1904, Plate 21, fig. 1), and young Phormosoraa (Plate 3, fig. 

 9), a single row of continuous ambulacral plates surrounds the mouth forming the peristome at 

 that age, but the initial single plate of the interambulacrum appears in the second row which 

 is the base of the corona, instead of the third row as in Bothriocidaris. In clypeastroids and 

 spatangoids (Collyrites, Plate 3, fig. 15) the initial single plate of the interambulacrum appears 

 intercalated between the plates of the primordial ambulacral row, which, therefore, are not 

 continuous as in regular Echini, but discontinuous, and, moreover, do not form part of the 

 peristome, but form with the primordial interambulacral plates the basicoronal row of plates 

 of the corona proper. 



Characters of Basicoronal Plates. 



Basicoronal plates are the oldest or first formed plates existent in any given corona, and 

 unless plates have been removed by resorption, or by flowing down on to the peristome, as in 

 the ambulacrals of cidarids, they are the actual plates formed at this area in the young animal. 

 Therefore, as elsewhere discussed, they often present simpler characters than plates dorsal 

 to them, and thus show stages in development. These plates on the proximal side give rise 

 to the attachment of part of the muscles of the Aristotle's lantern, either directly or through 

 the elevated processes of the perignathic girdle, which is later discussed. There are certain 

 types of structure in basicoronal plates which are comparatively constant in large groups, so 

 that they are of more than specific or generic value. 



The characters of basicoronal interambulacral plates are the more striking and may be 

 stated in brief. Where no plates have been removed by resorption, there is a single plate at 

 the ventral border of the interambulacrum. The primitive type of this character is Bothrio- 



