16 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



It seems probable that in these forms attachment was had in the 

 young stages by a central apical plate. Continued fixation eventually 

 caused a considerable constriction of the lower portion of the theca 

 even though attachment did not continue throughout the individual 

 lives of the animals. This condition of affairs has already been noted 

 in the case of Pyrocystis yirum. A continuation of the process would 

 eventually result in a defuiite orientation of the plates comprising 

 the pedicle, and a further differentiation of tliis portion of the animal. 



In Arachnocystis is to be noted a very considerable development 

 of the brachioles. These are fairly stout, and apparently have a 

 length of at least twice that of the theca. They are biserial, and 

 either two or three are borne by an individual. Taking into account 

 the probable buoyancy of the animal, it is not inconceivable that 

 these brachioles might have constituted fairly effective swimming 

 organs. 



The high interest that such a form holds is due in no small part 

 to the continued activity of a tendency in modifying an animal, even 

 though the conditions of life for which the modifications are fitted 

 are but imperfectly and irregularly effective. A more extended 

 discussion of this topic will be given in a subsequent portion of the 

 paper. 



Ascocystis. — In the remarkable genus Ascocystis, we have indubi- 

 table evidence of the nonattachment of the organism. The column 

 is extremely short, being but a fraction of the length of the theca. 

 It tapers rapidly and apparently ends in a point. There is absolutely 

 no evidence of fixation. This genus is equipped with a large number 

 of brachial appendages which completely encircle the distal portion 

 of the theca. It seems quite reasonable to expect that with its con- 

 siderable development of brachioles the genus may have been well 

 adapted to a free-swimming existence. 



Macrocy Stella. — Among the earliest of the forms referable to this 

 group, though by no means the most simple, is the curious genus 

 Macrocystella which might better perhaps be described as a tricyclic 

 crinoid than as a cystid. Figure 7, Plate 2, copied from Bather gives 

 a reconstruction of this remarkable Upper Cambrian genus. As 

 may be seen, the distal portion of the column is quite slender, and it 

 seems probable that it was used to loop about some stationary object. 

 The characters of the stem agree in all essential respects with those 

 given as typical of the majority of the members of this group. 



In connection with the eleutherozoic habit of this type is to be 

 noted the total absence of a column in the case of Lichenoides, a genus 

 probably closel}^ allied to Macrocystella, and referred to the same 

 family by Bather. 



Glyptocystis. — In the case of Glyptocystis a detached existence is 

 predicated largely on the basis of the column structure. It is to be 



