The opposed parting surfaces formed upon the bisection are spaced from each other to form an opening portion, and {TAIL ~ each side} of {HEAD ~ the polygonal rib} is so shaped that the cross sectional area of the opening portion is reduced as one goes from the opening end toward the depth 
The opposed parting surfaces formed upon the bisection are spaced from each other to form an opening portion , and { TAIL ~ each side } of { HEAD ~ the polygonal rib } is so shaped that the cross sectional area of the opening portion is reduced as one goes from the opening end toward the depth
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH