TERMINOLOGY. 



Some new terms are made use of and old terms are not used uniformly by all investigators, 

 so that the principal terms employed are given here in brief. 



Test: the whole skeleton including the spines; or without spines, denuded test. Fre- 

 quently test alone is applied to the skeleton in which spines are more or less wanting, the com- 

 mon condition in fossils. 



Corona: the portion of the test, including the ambulacral and interambulacral plates, 

 extending from the periphery of the peristome to the ventral margin of the apical disc. In 

 describing an ambulacrum or interambulacrum, the terms right or left are frequently used. 

 They mean the right or left side as seen facing the given area, viewed from the exterior and with 

 the dorsal portion uppermost. 



Ambitus: the zone of greatest circumference as viewed from above. The ambitus may 

 coincide with the mid-zone, Maccoya sphaerica (Plate 32, fig. 5), or it may lie ventral to the mid- 

 zone, Hyattechinus beecheri (Plate 24, fig. 7), or Clypeaster; or rarely dorsal to the median 

 zone, Lepidesthcs coreyi (Plate 66, figs. 8-10). 



A sea-urchin is divisible into a series of zones which theoretically are as many as there 

 are horizontal rows of plates, and each zone, at least in earlier life, differs or may differ in char- 

 acter from the preceding or succeeding zone. This is shown graphically in Hyattechinus 

 beecheri (Plate 26) ; here in the first zone of interambulacral plates there is one plate in each 

 area, in the second zone two plates, in the third three plates, and so on up to the introduction 

 of the full number of columns of plates, above which we reach zones of senescence, where plates 

 begin to drop out. In practice, radial variation interferes somewhat with the perfect zonal 

 expression, as all the areas do not develop with exactly the same degree of rapidity. For 

 example, in Lovenechinus missouricnsis (Plate 41, tig. 1), the fifth column originates in the fifth 

 row above the initial plate of the fourth column in area A, whereas in (he other areas it origi- 

 nates in the third or fourth row. Similar variations in rate of development are frequent in 

 later added columns, as shown (Jackson and Jaggar, 1896) in detailed stiulies of Melonechinus 

 multiporus, and as shown here in many figures. A column of plates may be quite wanting in 

 one area, though developed in other areas of the same specimen. In Hijattechimis rarispinus 

 (Plate 23, fig. 3) there are only eleven columns of plates in areas A and G, while there are 

 twelve in area I and thirteen in C and E. In Lovenechinus missouriensis (Plate 41, fig. 1) the 

 sixth column is represented by one or two plates in areas E, G, and I, i)ut is wanting in A and 

 C. A single zone is of course greatly modified in certain areas in bilateral types, as in ambu- 



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