UINTACRINUS: ITS STRUCTURE AND RELATIONS. 25 



with their arms interlaced. PI. III., Fig. 1 shows the two in adult speci- 

 mens, together with a young individual, in close contact. Other specimens 

 from this locality of Form M are shown on PI. V., Fig. 6, and PI. VL, Fig. 6, 

 and of Form D on PI. III., Fig. 2 ; PI. V., Fig. 7; PI. VI., Figs. 1 and 2. 



The photograph on Plate YIIl. also shows the two forms in mature 

 specimens occurring close together on the slab. 



Both forms are also found in the colony from Locality No. 2, where 



out of 99 specimens showing the base — 



24 belong to -Form M = 25 per cent 

 75 " " " D = 75 " " 



Specimens of Form M from this locality are illustrated on PI. III., Figs. 

 3 and 5, and of Form D on PI. III., Fig. 4. 



The two forms also occurred together at Locahty No. 6, where speci- 

 mens showing the base are rare in the material as obtained. I have ob- 

 served three belonging to Form M and one belonging to Form D, — all 

 adult specimens. 



The differences presented by Forms M and D are not correlated with 

 any other characters. They have no apparent relation to size or maturity 

 of the specimens, and therefore could not have been the result of differ- 

 ences in individual growth. The monocyclic form is found among the 

 youngest specimens (PI. III., Fig. 5) as well as the oldest; and the same 

 thing is true of the dicyclic form (PL III., Fig. 4). Neither are they con- 

 nected with variations in the interbrachial areas. I shall near the end 

 of this paper give a tabulated statement of observations in these various 

 particulars on more than five hundred specimens, by which the validity of 

 the above assertions may be tested. Both forms are found in the same 

 colonies, where all are inextricably tangled together ; and I have no 

 doubt that those in each colony floated together as one mass, and belonged 

 to the same species. 



Nevertheless, there are two distinct types of base among them. It is 

 not a case where the infrabasals are concealed, or are more or less perfectly 

 or imperfectly developed, or are fused with some other element, — as in 

 some forms which Bather calls pseudomonocyclic. For here Wachsmuth 

 and Springer's law of alternate arrangement of the elements of the Crinoid 

 skeleton strictly prevails : — 



a. When the infrabasals are wanting (Form M), the centrale is radial 

 in position. 



