It is normal .It is Azygus fissure which subtends a variable amount of the superomedial portion of RUL to form Azygus lobe as nicely described by BENJAMIN FELSON .

It is an anomalous fissure present in 1% of anatomic specimens and is visible in about 0.4 % of chest x-rays.It differs from other anomalous fissures as it bears no constant relationship to the anatomic segments of lung .

Development :Azygus fissure is considered to be related to embreyonic descent of heart from its cervical to its thoracic position before the azygus vein has reached its normal position .the paired folds of parietal &visceral pleura form a mesentry like structure, the mesoazygus .

The comma shaped ,end -on shadow of azygus vein , normally positioned in the crotch between the upper lobe bronchus & the trchea , falls short in its migration and lies well within the lung . At the origin of the mesoazygus on the internal thoracic wall , there is a triangular shadow known as the trigonum parietale .

Other anomalous lobes are superior accesory lobe ,inferior accesory lobe & very rarely left azygus lobe .