4 Trans. Acad. Sci. of St. Louis. 



bipyramidalis, Hall (Saccoblastus), we have five fork pieces 

 but six ambulacra, of which two occupy one sinus. I think 

 these examples are sufficient to justify the above statement. 



Fig. 1. 



Fig. 2. 



The body of a Pentremite is constructed of twenty pieces, 

 (at least in the embryonic state), arranged in two circles in 

 such a way that we have a most perfect dicyclical* body 

 with a primary center of a pentagonal outline for each circle. 

 Each piece of the second circle rests upon two of the first 

 circle. These two circles may be designated as an ambulacral 

 and an interambulacral, of which the former grows down- 

 ward in its development, whereas the latter grows upward. 

 Fig. 1 and 2. 



THE BASAL PIECES. 



The base or central part of the interambulacral circle (also 

 called pelvis )t varies very much in size and in general form, 

 from a flat disc to a more or less funnel-shaped piece. It is 

 perforated by a fine channel in the center, and the articula- 

 tion surface for the column is either round or triangular ac- 

 cording to the species and consists, at least in the embryonic 

 state, of five equal pieces, though in the course of development 

 two of the five sutures become obliterated, so that it gener- 



♦ The word dicyclical is not used in the same sense as in Crinoideae. 

 t H. B. Geinitz. Grundriss der Versteineruugskunde, p. 558. 



