NO. 184G. ON CERTAIN ELETJTHEROZOIC PELMATOZOA—KIRK. 27 



the theca. As a result, the possibihties of vertical movement were 

 far more limited than in the case of the later types. It seems reason- 

 able to expect that even in Trochocystis the column performed no 

 slight service in the propulsion of the animal. If Jaekel (1901) be 

 correct m assuming the presence of marginal ambulacral structures 

 it might follow that movement was to some extent performed by 

 brachioles or their equivalents. The presence of food-grooves in this 

 type I feel is far from proved, however. 



In numerous figures given of Trochocystis by Barrande (1887) is 

 to be noted a curious central distal plate, of which an enlargement is 

 here given on Plate 3, figure 4. This plate is at times extended 

 beyond the theca, and at times folded down apparently covering the 

 central anterior aperture. The structure of this plate and its appar- 

 ent relation to the theca at once suggest a function similar to that of 

 the hinged plate described in the case of Anomalocystis. It seems 

 probable indeed that we are here dealing with homologous struc- 

 tures. If such be the case, the surface to which the plate is hinged 

 m Trochocystis corresponds to the dorsal side of Anomalocystis. 



Concerning the terminal openings of Trochocystis but little can defi- 

 nitely be said. There certamly is a central aperture corresponding 

 essentially with that of later types. Barrande (1887), Haeckel (1896), 

 and Bather (1900) state that there are two other openings, one on 

 either side of this central aperture. Jaekel (1901) admits of but one 

 other opening, to which he claims the ambulacral grooves run, thus 

 proving it to be the mouth. Doubtless the question mil not be 

 settled until someone makes satisfactory impressions from external 

 molds. It would seem more reasonable on the whole to consider the 

 central openuig the mouth, although this view is at variance with the 

 stand taken by former writers w^ho have presumably had an oppor- 

 tunity to study good material. 



Mitrocystis. — In Mitrocystis, Plate 3, figures 6-8, is to be found a 

 type intermediate in structure between Trochocystis and the later 

 genera of the group. In all essential details the genus approximates 

 more nearly to the later forms than it does to Trochocystis. 



There is to be noted a marked differentiation into an upper (dorsal) 

 and a lower (ventral) surface. In the ventral surface there has been 

 a v^y considerable consolidation of the plates, the number of those 

 mclosed in the circumference of the marginals numbering but a half 

 dozen at most. The dorsal surface is still composed of numerous 

 iiTegularly arranged plates, although they are fewer in number than 

 in the case of Trochocystis. Bilateral symmetry in respect to a 

 vertical longitudinal plane is as yet scarcely appreciable. 



Perhaps the most notable structural difference between Mitro- 

 cystis and Trochocystis lies in the fact that Mitrocystis has but a 

 single terminal aperture, therein agreeing with the Silurian and 



