10 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



plaquettes, qui constituent leur extremite inferieure. Celle-ci ne presente point la 

 cavite simulant une ouverture, que nous avons figuree en divers specimens sur la 

 PL 12, et qui appartiennent au type: Aristoc. Bohemicus. 



It is to be noted in the case of these specimens that there is no basal 

 depressed area, no marked asymmetry of the theca, and the plates 

 all show clearly defined diplopores, Fm'thermore, the central apical 

 plate is sometimes wanting, and when present is frequently fused to 

 one of the circumjacent plates of the proximal ring. The plates are 

 tumid, and fixation may only be predicated at a very remote stage 

 in the ontogeny of the animals. AHstocystis desiratus figured by 

 Barrande on Plate 20, figm-es 1 and 2, indicates a very similar condi- 

 tion of affairs. 



Craterina. — In the case of Craterina evidence of non-attachment 

 similar in all respects to that just given respecting AHstocystis might 

 be adduced, although as a rule fixation seems to have been maintained 

 until a relatively later stage in development. In a number of in- 

 stances the pit indicating the former area of attachment has tubercles 

 over the surface which obviously could not have been present at the 

 time of fixation, and must consequently have been formed subsequent 

 to the detachment of the animals. 



Pyrocystis. — Pyrocystis likewise furnishes conclusive evidence that 

 in its adult stage fixation was but irregularly effective. A specimen 

 of Pyrocystis pirum figured by Barrande (PL 29, II, figs. 23, and 24) 

 shows no signs of having been cemented to the bottom. The base of 

 the theca is rounded; there is no impressed area, and the entire sur- 

 face is quite strongly tubercular. The assumption of freedom by this 

 type is somewhat remarkable in that the general form of the theca 

 indicates a fairly well established statozoic habit. The marked con- 

 striction of the lower portion of the theca which is the precursor of a 

 differentiated pedicle or column, and the symmetrical arrangement 

 of the food grooves point most strongly to this conclusion. 



Orocystis. — The placing of Orocystis in this group rests upon some- 

 what uncertain evidence. In a view of the basal portion of a speci- 

 men given by Barrande (PL 7, fig. 15) there is no sign of a point of 

 attachment. Barrande points out this fact, but says "nous croyons 

 reconnaitre sa place." Without an exammation of the specimen 

 itself it is impossible to determine the status of the form. The chances 

 are, however, that if the point of fixation were so inconspicuous as 

 not to be indicated in the figure, we are safe in assigning Orocystis 

 Jielmhaclceri to the eleutherozoic Cystidea. Fixation if effective in 

 such a form, at least in the adult stage, would leave an unmistakable 

 impress on the structure of the organism. Considering the remark- 

 ably fine preservation of the material, any evidence of fixation should 

 be at once apparent. 



