52 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



Possible appearance of infrahasals externally in Millericrinus and 

 Apiocrinus. — A very interesting feature is shown by certain of these 

 specimens, as, for example, figures 1, 3, and 4, in the appearance 

 within the cup of certain apparently incongruous plates. Similar 

 plates are to be noted in Apiocrinus, as shown in figures 7 and 8, 

 which are copied from de Loriol. These represent A. elegans and A. 

 roissyanus. The presence of such plates has been noted from the 

 earliest times and by a considerable number of writers. No one 

 apparently has endeavored to account for them except as being abnor- 

 malities. Carpenter (1882, p. 35) says: 



I can form no idea as to the meaning of these accessory plates; they are evidently 

 without any morphological importance, or they would be more constant in their 

 occurrence. 



As here held, these plates have a definite morphic significance. In 

 brief, I consider them to be infrabasals. The curious occurrence of 

 these plates is due to the structure of the base. It will be noted in 

 figure 12 as given by Carpenter, or in any figures of the proximal 

 columnal of Apiocrinus figured by various authors, that this ossicle 

 extends far up within the cavity formed by the basals. The infra- 

 basals where observed lie, as they necessarily should, at the extreme 

 apex of this subpyramidal plate. Tliis gives them a horizontal posi- 

 tion of approximately that of the top of the basals, or even higher. 

 It may readily be seen if for any reason these infrabasals should 

 become hypertrophied and extend outward to the surface of the 

 theca that they would appear either between the basals or at the 

 juncture of the basals and radials. The extension of these plates 

 downward so as to come in contact with the proximal columnal in an 

 external view may perhaps be considered a secondary feature. How- 

 ever, this condition might be reversed were the infrabasals to extend 

 outward at a much earlier stage in the ontogeny of the crinoid. This 

 may account for the conditions as we find them in Apiocrinus rois- 

 syanus (PI. 6, fig. 7), although one hesitates to deny that the struc- 

 ture here observed may represent the formation of a new proximal 

 columnal, after the manner of Millericrinus prattii. 



This hypothesis relative to the identity of these plates with the 

 infrabasals is not perhaps capable of demonstration without the care- 

 ful dissection of a theca showing such structures. It is, however, 

 inconceivable that the plates can represent anything else. The rea- 

 son for such an appearance of the infrabasals is not obvious. The 

 sporadic appearance of entirely new plates is even more inexplicable, 

 however. It must be borne in mind that the forms wliich show such 

 plates are near the ends of their respective lines, and it is among such 

 types that unusual structural features are apt to appear. 



The relationships of Millericrinus. — The relationships of Milleri- 

 crinus have received but scant attention, particularly as regards the 



