As mentioned before, the droprate on these shells is way crappy. Me and my friend were here for about 1.5-2 hours and got 3. We were going for 6 though. Now we have to go back again. Also, respawn on these is about 15 mins which makes it even more of a pain in the ***** Best thing to do is try to get a coffer key and search for a coffer while waiting on respawn.
the shells are EX, end of story, you can NOT buy them, trade the, bazaar them or AH them. They only drop off of starmites which spawn in 1 circular room in the bowels of the canal. They're actually good skill up mobs and you could lvl NPC off of them too since they're 65-67 which is what 2 friends and i ended up doing after we realized how bad the drop rate was. Made the time go by faster.
Some things one should know about farming these shells is that the drop rate is not too common (or that rare). Approximately 1 in 5 but would vary with moon phase and Treasure Hunter.
As well if one plans to solo these mobs be careful of the Dire Bats that roam the halls adjacient to (G-9) where the Starmites are. There is however a small area between the starmite room and the main hall where you can stay aggro free 95% of the time but requires trial and error to line it up.
Might be worth it to hunt for the Coffer key off these mobs for the Wild Card quest while your at it, although Coffer key drops off the starmites are less common than on the Skeletons.