the same number of depressions. As regards the edge hooks^they lie 

 along the exterior edge at the places where it changes into the secondary- 

 septa, little altering their positions in connparison with those of the 

 preceding group (Fig. 274, D). Such a type of structure of the disc is 

 characteristic for Merizocotyle . 



Further reduction of the initial ring is characteristic for the 

 following two genera of Monocotylidae-- Thauniatocotyle and Empruthotrema , 

 specifically that both middle transversal septa are also subjected to reduction 

 as a result of which there remains only 5 septa and 5 depressions. With 

 this one cannot fail to note, as is clear from the diagrams, (Figs. 274, 

 E and F) that the two upper septa remain unchanged while the reduction 

 of the septa proceeds, so to speak, from the lower edge of the disc. As 

 regards the secondary ring of septa and depressions, their structure is 

 clear from the diagram cited. Apparently one can consider that here took 

 place only the bifurcation of the septum of the original ring ( Empruthotrema ) 

 with certain complications among Thaumatocotyle . The nature of the dis- 

 tribution of the edge hooks apparently remains the same, but as we did 

 not have our own material we can only say for sure that their correlations 

 with tie middle hooks are retained, i. e. , 4 hooks lie between the latter and 

 10 above them. The reduction of the middle hooks among Empruthotrema. 

 does not alter these correlations (the 4 lowest edge hooks lie between the 

 two lowest septa). 



Finally, the greatest complication is attained by the structure 

 of the disc among Cathariotrema where the formation of a large number of 

 secondary septa and depressions takes place (Fig, 47). Unfortunately, it 

 is impossible to say anything about the nature of these transformations 

 because we did not have the material,and the authors (MacCallum 1916a; 

 Price, 1938a) who studied this form did not attach significance to the 

 accurate representation of the disc and the counting of the number of 

 separate septa or depressions. 



The morphological changes of the disc which we have just p. 367 



described show that the general direction of the evolutionary process in 

 the given group proceeds along the lines of fragmentation of single 

 sectors of the disc into a larger or smaller number of morphological 

 and functional sections which apparently gives a greater adhesive affect 

 in total than the action of single sucker. One can suppose that this is also 

 true for all the remaining groups which have the attaching apparatus in 

 the shape of a sucker or a system of suckers. 



436 



