28 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



Devonian genera. The uncertainty that obtains relative to the 

 nature of the openings of TrocJiocystis has been noted above. What- 

 ever be the structure in that genus, it is quite certain that we have 

 here an aperture that represents the confluence of the openings of 

 TrocJiocystis, if, indeed, there be more than one. It is interesting to 

 observe the presence of the central, hinged plate in the case of Mitro- 

 cystis. In Plate 3, figure 8, which represents the under (imier) sur- 

 face of the plate, there are to be noted several longitudinal ridges. 

 The grooves between these ridges may well represent the points of 

 attachment for the muscles that move the plate. 



The column of Mitrocystis seems to be in all essential regards quite 

 similar to that possessed by the later genera referable to the Ano- 

 malocystidse. Bather (1900, p. 51) describes it in part as follows: 

 "Stem of about four alternating rows of plates, often provided with 

 thorn-like processes." Barrande's figures do not seem to justify this 

 portion of the description. The column apparently is composed of 

 the normal, double row of ossicles, which seem to be apposed in many 

 cases instead of alternating. In some instances indeed partial 

 fusion seems to have taken place, such as has been described in the 

 case of Anomalocystis. The thorn-like processes described by 

 Bather are supposed to be shown in Plate 3, figure 6, which is copied 

 from Bather (1900), who in turn copied it from Barrande. These 

 processes, I believe, are more apparent than real, and are due to the 

 preservation of the specimen rather than to the actual structure of 

 the column. Other specimens illustrated by Barrande show no 

 signs of such spines and agree with allied genera in this regard. It is 

 evident, I think, that in the case of this specimen the substance of the 

 plates has been removed and the processes, if they represent any- 

 thing, are ridges of ferruginous matter deposited by infiltration in the 

 stem cavity and between the segments. 



The articulation of the column with the theca in Mitrocystis repre- 

 sents a decided step in advance over the structure shown in the case 

 of TrocJiocystis. There is apparently a fair invagination of the proxi- 

 mal portion of the theca for the reception of the column, and the 

 stem may well have had considerable freedom of motion both in a 

 horizontal and vertical direction. 



General discussion of type 5.— There are several other genera refer- 

 able to this group, but inasmuch as they exhibit no special modifica- 

 tions of structure other than those shown by the forms described, it 

 has not been thought best to include them in the present discussion. 



The essential unity in structure shown by the genera referred to 

 this group is notable and argues strongly in favor of their monophy- 

 letic origin. Derived no doubt, as suggested by Bather (1900), from 

 a form not widely dissimilar to Aristocystis, a prostrate habit of life 

 was early assumed and constantly maintained. In consequence the 



