In typical bivalves two adductor muscle scars (anterior and posterior) are present, although in some species one of these may be greatly reduced in size or even absent. Joining the two adductor muscle scars is the pallial line. This marks where the pallial muscles fasten the mantle to the shell. The pallial sinus, when present, marks the insertion of the muscles used to pull the siphons into the shell. If the siphons are not retractable or are very short, there is no pallial sinus. However, in deep-burrowing forms with long siphons, the pallial sinus is extremely pronounced.