Fissurella and Drymaeus (Mexico)?

[wolf]

Hi,
yesterday evening I was very happy to find this excellent "high end" forum and I will be glad to join it. I´m from Germany, so please apologize my terrible English. Concerning gastropods, I´m an absolute beginner.
I´m just musing on two shells from Mexico, perhaps someone can help me along.
The first species might be <i>Fissurella barbadensis</i> (GMELIN, 1791). Both specimen (with intensive greenish colour on the inside) were found at Xcacel (east side of peninsula Yucatán, State of Quintana Roo, 23.07.1999), length of shell 18 and 21 mm. I´m not sure about the curious "furred" structure of the surface (whether it might be some strange periostracum or produced by some other organism). The other shell of the same species is free of it.  
<img src="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/FissBarbPUFo1.jpg" alt="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/FissBarbPUFo1.jpg" style="border:0" /><img src="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/FissBarbPO1Fo1.jpg" alt="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/FissBarbPO1Fo1.jpg" style="border:0" />
<img src="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/FissBarbSO1Fo1.jpg" alt="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/FissBarbSO1Fo1.jpg" style="border:0" /><img src="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/FissBarbSSFo3.jpg" alt="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/FissBarbSSFo3.jpg" style="border:0" />
The other species might be <i>Drymaeus serperastrum</i> (SAY, 1829), hight 34,6 mm (Mexico, State of Yucatán, Valladolid, Cenote Zaki, 26.07.1999). Could it be right?
<img src="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/DrymaeusSerperastrumFo1.jpg" alt="http://i803.photobucket.com/albums/yy320/wolfsnail/Mexiko1/DrymaeusSerperastrumFo1.jpg" style="border:0" />
Thanks a lot for your help <img src="images/smiley/smile.gif" alt="" border="0" />, kind regards: wolf