152 SYNOPTIC COLLECTION. 



just above the stem is composed of basals and the circle 

 above of radials. The other genera have three circles of 

 plates; the one at the base of the body and above the 

 stern is the circle of underbasals and above this are the 

 basals and radials. 



Those forms which have basals and no underbasals are 

 known as monocyclic Crinoids and those with both basals 

 and underbasals as dicyclic forms. 



According to some investigators the monocyclic Cri- 

 noids of recent geologic epochs and those living to-day 

 have descended from the ancient dicyclic forms. If this 

 is the case, they have become specialized by reduction. 

 While this is probably true, it must be borne in mind 

 that the ancient dicyclic forms may have arisen from prim- 

 itive monocyclic Crinoids, which one would expect to find 

 in pre-Cambrian formations. 



According to Bather and Lankester, 1 there is evidence 

 that the monocyclic forms have descended from Cambrian 

 or pre-Cambrian monocyclic ancestors, and the dicyclic 

 species from dicyclic ancestors, though it is not known in 

 what forms these two independent lines converge. We 

 will consider Haplocrinus as an example of the ancient 

 monocyclic group. Its body (PI. 266, figs. 1-3, H. mespi- 

 liformis) is small and attached by a round stem not seen 

 in the drawings. It is composed of basals and radials, 

 as we have already said, and these plates are fastened 

 together by close sutures, so that they are immovable (fig. 

 i, side view). The radials are perforated. The ventral 

 pyramid consists of oral plates only, 2 which rest upon the 

 radials. Fig. 2 is the ventral surface with the pyramid of 

 five oral plates; the posterior plate which contains the 

 anus is seen to be larger and is carried forward between 

 the lateral-anterior plates covering the mouth. Some- 



1 Treat. Zool., Part III, 1900, p. 138. 



2 Wachsmuth and Springer, Proc. Acad. Nat. Sci. Phila., 1888; 

 also ibid., 1890. 



