208 PROCEEDINGS OF THE PALEONTOLOGICAL SOCIETY 



mary simplicity of structure of the genotype in the presence of more or less 

 numerous intercalated plates, resulting in a considerable variety of patterns 

 in the mature specimens. Young specimens of this species, however, probably 

 were quite typical in appearance. This is suggested by the fact that if the 

 margins of all plates are reduced at the same rate, the smaller plates will 

 disappear first, and then those next in size, until only the primary plates 

 remain. These primary plates in that case, moreover, will then be arranged 

 in transverse rows, eight in a row, in such a manner that if the plates were 

 brought closer together those in successive rows would alternate with each 

 other, and their interference on growth would tend to produce hexagonal 

 outlines, as in Holocystites cylindricus. 



Between the first and second transverse rows beneath the circum-oral plates, 

 intercalation frequently is confined to single plates, inserted at the angles of 

 the primary plates, resulting in pentagonal outlines in the intercalated plates. 

 Additional intercalation at the apex of the latter results in the truncation of 

 the latter, producing an hexagonal outline, the second intercalated plate being 

 triangular. Similar simplicity of structure may occur between the second and 

 third transverse rows of plates, and even between the third and fourth rows, 

 counting downward from the top. 



Beneath the third transverse row, in some specimens, intercalation may 

 take place at three successive intervals, resulting in a more or less compli- 

 cated pattern of small plates. Much less frequently complicated patterns of 

 small plates occur also between the second and third transverse rows of plates 

 and even between the first and second rows. 



The primary plates of each transverse row usually remain in contact with 

 each other laterally or are separated by two intercalated plates, one of which 

 has wedged in from above and the other from below. In other words, addi- 

 tional plates appear in much greater numbers between the transverse rows 

 of primary plates than between the adjacent primary plates of the same row. 

 The growth of the animal within the theca appears to have produced tension 

 chiefly in the axial direction, resulting, in Holoy ostites alternatus, in frequent 

 intercalation of plates, especially toward the base in some specimens. This 

 resulted in a theca which was conspicuously elongated, especially toward the 

 base. 



In typical Holocystites ahnormis a similar elongation of the theca was pro- 

 duced by the elongation of the individual primary plates without the addition 

 of intercalated plates. 



If Holocystites scutellatus, Holocystites ovatus, and Holocystites ivinchelU 

 are to be regarded as genuine species of Holocystites, it is evident that con- 

 siderable variation in the plate diagrams occurs also in each of these species, 

 accompanied more or less with intercalation of additional plates. 



It then becomes evident that the discrimination of species of Holocystites, 

 in the presence of numerous specimens, is likely to prove a matter of more or 

 less difiiculty, especially if specific differences be established chiefly on the 

 general appearance of the plate diagram and the form of the theca as a whole. 

 In such cases it is probable that a much safer guide will be found in the 

 ornamentation of the individual plates of the theca, such as a central boss, 

 radiating ridges, the presence, size, and arrangement of the granules, and 

 the like. 



