PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



figure 11, has a subglobose theca composed of a great number of" 

 irregular, polygonal plates. These plates are remarkably thin, and, 

 considering the size of the animal, the skeletal structure must have 

 been exceptionally light. In a Russian specimen of average size the 

 plates measured but 0.25 mm. in thickness. Even this does not rep- 

 resent solid stereom, as the substance of the plates is traversed by 

 innumerable series of radiating tubes. The brachial appendages were 

 no doubt comparatively slender and weak. 



In the aboral portion of the theca there is usually a slight projection, 

 which when present doubtless served as a point of attachment for the 

 organism. Tliis protuberance is apparently an evagination of the 

 body wall and seems to be restricted to no closely circumscribed area 

 as to location, other than that it is near the aboral pole. So far as the 

 descriptions of the species are concerned, this projection seems to be 

 constantly present in European specimens. In the American forms, 

 which are apparently specifically identical with the European, the 

 protuberance is apparently wanting at times, but a cicatrix marks its 

 former location. There are signs of resorption and secondary depo- 

 sition of stereom, indicating that although attached at one time the 

 animals had subsequently become free. 



Even when present, this projection, because of its relatively insig- 

 nificant size, seems quite inadequate as a support for an adult animal. 

 So far as I have been able to see in the specimens I have examined the 

 distal end of the projection shows no signs of attachment such as one 

 would expect to find were the animals cemented to the bottom at the 

 time of their death. Rather, the free end seems irregularly broken, 

 but not exliibiting in many cases the clearly defined angles of a fresh 

 fracture. Furthermore there is no considerable thickening of the 

 plates of the pedicle as preserved and no obliteration of sutures, 

 features that are usually characteristic of that portion of a Pelma- 

 tozoan lying in immediate proximity to the point of cementation. 



Again, if the adult cystids were attached by means of such a short 

 pillar, it seems probable that the area immediately surrounding the 

 point of attachment would be more or less in contact with the bottom 

 and would show signs of such apposition. This is especially to be 

 looked for in cases where the point of attachment is asymmetrically 

 located at some distance from the aboral pole as in Plate 1, figure 

 11. No signs of abrasion or contact modifications are to be noted, 

 however. 



In other many-plated Cystidea that were unquestionably affixed 

 to the bottom in their adult stages evident signs of attachment are 

 to be observed. An examination of the figures of Aristocystis, 

 Fungocystis, and Craterina as given by Barrande clearly shows the 

 effect of such cementation. In Aristocystis hohemicus, Plate 1, 

 figures 1, 2, the animal is cemented to a gastropod shell which it has 



