18 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



The more obvious features to be noted are: The acquisition of a 

 marked asymmetry, the theca being differentiated into an upper and 

 a lower surface ; the locahzation of pore-rhombs on one (the upper) 

 surface; the atrophy of certain food grooves; and finally the shifting 

 of the anal opening to the extreme proximal portion of the theca. 

 These features may be present in varying degrees, and are usually 

 accompanied by characterisitic minor phenomena. It is to be noted 

 that these modifications are even more characteristic of those forms 

 constituting type 3. There, however, the same results are often 

 attained by different methods. 



Dendrocystis. — In Dendrocystis we have a very primitive type 

 which may well be referred to the prostrate division of tliis group. 

 A close comparison of the figures given by Barrande shows that we 

 may differentiate between an upper and lower surface of the theca. 

 Moreover, the anal aperture has been carried well backward, and is 

 situated in the extreme proximal portion of the theca. 



But a single brachiole is borne by the animal. This is remarkably 

 stout, and doubtless served in the propulsion of the animal. It has 

 been considered by Barrande and others that this organ is a closed 

 tube. Bather, on the other hand, believes that a ventral groove was 

 present, wliich is as a rule closed by the apposition of the lateral rows 

 of plates forming the brachiole. It seems highly probable that the 

 latter view is correct. 



The theca of Dendrocystis is composed of a great number of irregu- 

 larly arranged plates. Nevertheless it seems possible to distinguish 

 an upper and lower surface. Barrande's figures indicate rather 

 diverse structural conditions, which may be due to actual variation 

 within the species-, or, what seems fairly probable, to the fact that 

 there are two species represented. For this reason and because of 

 the fact that I have been able to examine but indifferent original 

 material, I am unwilling to draw other than tentative conclusions in 

 regard to the form. 



The column in its proximal portion is made up of a great number 

 of irregularly arranged plates. Distad the plates become larger, and 

 a more definite arrangement is to be noted. The greater part of 

 the column is slender and composed of a double series of elongate, 

 alternating plates. This arrangement indicates a considerable de- 

 gree of flexibility. Such a column could not have been effective 

 othor than as an, organ by which temporary attachment might be 

 had through the looping of its distal portion about some stationary 

 object. 



Before discussing the typical prostrate form Pleurocystis, attention 

 should be called to a few genera that show a more or less clearly 

 defined tendency toward the assumption of such a form of existence. 



