Hnmbach —Revision of the Blastoideae. 5 



ally appears as if the base were constructed of only three 

 pieces, of which two are equal and pentagonal and the other 

 rhombic in form, though specimens are fre- 

 quently found in which no anchylosis took 

 place so that all five sutures remain open, an 

 occurrence which is also observed in Crinoids, 

 for example in Agassizocrinus where the base 

 generally appears as one solid piece, though fig. 3. 

 specimens are not uncommon where one, two or all sutures, 

 remained open and did not anchylose. Fig. 3. 



FORK PIECES. 



The fork pieces are of an oblong form, more or less wide, 

 according to species, and the incision or sinus is also of vari- 

 able depth and width, making them resemble a two-pronged 

 fork, from which the name originated, or a V shape. Their 

 solid base portion is more or less thick and has, in cross sec- 

 tion, a more or less triangular or semi-lunar form, or, as in 

 other genera of the family Pentremidae, it is turned in and 

 upward so that the base portion becomes inverted and exter- 

 nally invisible. The incision or sinus is wider externally 

 than internally, i. e., the inner lateral margin of the sinus 

 slopes toward the interior, causing the internal opening of the 

 sinus to be narrower than the external. The upper* points of 

 the prongs are cut obliquely laterally making them rest against 

 the upper and lateral spurs of the deltoid expansion. In 

 other species they run out into sharp and acute points as in 

 Pentremites reinwardtii , Troost. In still others they are of 

 uniform thickness with an upper oblique and smooth margin. 



THE DELTOID PIECES. • 



The deltoid pieces of the typical species, as Pentremites 

 sulcatus, i^yriformis, Jtorealis, etc., when fully developed, 

 can be divided into two parts, which, however, are not 

 separated from each other by sutures. First a main or 

 base portion, the most important part of the whole calyx. 



