another example the holes 40 had a minimum diameter of 0 062 inch and their spacing circumferentially and axially of the screen member was respectively 0 250 inch and 0 219 inch with each inlet portion 0 030 inch deep at its inner periphery the outer diameter of each inlet portion was 0 224 inch which provided a maximum overlap of 0 102 inch spirally of the screening member and a maximum overlap of 0 005 inch circumferentially of the screening member the combined total of the countersunk areas u03c0r 2 n would theoretically exceed the area of surface 45 except for the overlaps in another example the holes 40 had a minimum diameter of 0 125 inch and their spacing circumferentially and axially of the screen member was respectively 0 250 inch and 0 375 inch with each inlet portion 0 030 inch deep at its inner periphery the outer diameter of each inlet portion was 0 27 inch which provided an overlap of 0 020 inch between adjacent inlet portions axially of the screening member and 0 045 inch spirally of the screening member and a ridge 46 having a minimum width of 0 10 inch between adjacent inlet portions axially of the screening member the total of the countersunk areas u03c0r 2 n would again theoretically exceed the area of surface 45.