26 ROBERT TRACY JACKSON ON ECHINI. 



lacrum III in many spatangoids. Such extreme zonal modification does not occur in normal 

 regular Echini. 



The term zones has been sometimes applied as "ambulacral zones," but this seems an 

 unfortunate use of the word, for such areas do not surround the test, as a zone should, and 

 moreover are not parallel to the equator, which also the term commonly implies. Three main 

 zones may be recognized in any sea-urchin, which I would designate as the basicoronal zone, 

 mid-zone, and placogenous zone. In the basicoronal zone occur the basicoronal plates, the 

 first row of ambulacral and interambulacral plates of the corona bordering on the peristome or 

 buccal membrane. Loven called these plates peristomal plates, but it seems an undesirable 

 use of the word, which is usually applied to the buccal membrane and plates on the same. 

 I therefore introduce the term basicoronal as meaning exactly what is intended, and not to be 

 confused with terms applied to other parts. The basicoronal plates are of special interest as 

 being the oldest or first formed plates occurring in any given corona, and also they are the 

 plates giving support directly or through the perignathic girdle to important muscles of the 

 lantern (text-figs. 22-31, p. 70; and 221-230, p. 193). 



The median zone, or mid-zone for brevity, is that horizontal zone measured halfway between 

 the poles; it may coincide with the ambitus or lie dorsal to it, or rarely ventral to it. The mid- 

 zone is important in Palaeozoic and most other Echini, as the area in which in the ambulacra 

 and interambulacra the differential specific characters are typically most fully developed. 

 Ventral to it the full features may not have been attained, and dorsal to it we soon get into the 

 area of localized development, where the young plates have not attained the full characters 

 of the species. In clypeastroids and spatangoids that have petaloid areas, these lie dorsal to 

 the mid-zone, so that in such types special consideration of the dorsal area is necessary, but it 

 does not invalidate the mid-zone as an area of importance in the consideration of structure. 



The placogenous zone (■"'^af = plate, + yeved = birth) is that portion of the corona 

 next to the apical disc in which the newly added coronal plates occur both in the ambulacrum 

 and the interambulacrum. This zone is of importance as the plates in it quite generally present 

 youthful characters, which, as localized stages in development, can be compared with the 

 characters seen in the young, or in the adults of simpler tj'pes in the group (text-fig. 237, p. 231). 



Roiv, as applied to ambulacral and interambulacral plates, refers to plates lying in one 

 horizontal plane or zone, in contradistinction to columns. 



Column, as applied to ambulacral and interambulacral plates, refers to plates lying super- 

 posed in a vertical series. The column is of course made up of nothing other than plates of a 

 succession of rows, but is a convenient distinction to maintain in Palaeozoic types, where we 

 find in various forms from two to twenty columns of ambulacral or one to fourteen columns of 

 interambulacral plates. The column, especially in the interambulacrum, usually consists of 

 a very definite succession of plates forced into a continuous series by the mechanical impact 



