diff --git a/EgtGeomKernel.rc b/EgtGeomKernel.rc
index 95e9d6f..b1faae2 100644
Binary files a/EgtGeomKernel.rc and b/EgtGeomKernel.rc differ
diff --git a/SurfTriMesh.cpp b/SurfTriMesh.cpp
index 1a80f28..38b33ea 100644
--- a/SurfTriMesh.cpp
+++ b/SurfTriMesh.cpp
@@ -3810,12 +3810,19 @@ SurfTriMesh::VerifyConnection( bool bShellsAndParts) const
       BBox3d b3Box ;
       PtrOwner<ISurfTriMesh> pStmShell ;
    } ;
+  // vettore di shell con vettori dei propri triangoli (per accelerare nei casi degeneri)  
+   INTMATRIX mShellTria( m_nShells) ;
+   for ( int i = 0 ; i < ssize( m_vTria) ; ++ i) {
+      if ( m_vTria[i].nShell < m_nShells)
+         mShellTria[m_vTria[i].nShell].push_back( i) ;
+   }
+  // classificazione delle shell
    vector<SHELLINFO> vOuterShells ;
    vector<SHELLINFO> vInnerShells ;
    INTVECTOR vOpenShells ;
    for ( int nSh = 0 ; nSh < m_nShells ; ++ nSh) {
      // se la shell è chiusa
-      if ( IsShellClosed( nSh)) {
+      if ( ssize( mShellTria[nSh]) >= 4  &&  IsShellClosed( nSh)) {
         // creo una superficie clonata dalla shell
          PtrOwner<ISurfTriMesh> pStmShell( CloneShell( nSh)) ;
          if ( IsNull( pStmShell)  ||  ! pStmShell->IsValid())
@@ -3827,7 +3834,9 @@ SurfTriMesh::VerifyConnection( bool bShellsAndParts) const
          BBox3d b3Box ;
          pStmShell->GetLocalBBox( b3Box, BBF_STANDARD) ;
         // la inserisco nel vettore opportuno
-         if ( dVol > 0)
+         if ( abs( dVol) < 1 * 1 * EPS_SMALL)
+            vOpenShells.push_back( nSh) ;
+         else if ( dVol > 0)
             vOuterShells.emplace_back( nSh, dVol, b3Box, Release( pStmShell)) ;
          else
             vInnerShells.emplace_back( nSh, dVol, b3Box, Release( pStmShell)) ;
@@ -3979,6 +3988,7 @@ SurfTriMesh::IsShellClosed( int nShell) const
       return false ;
   // ciclo sui triangoli della shell
    bool bClosed = true ;
+   int nTriaCnt = 0 ;
    for ( int i = 0 ; i < GetTriangleSize() ; ++ i) {
      // se triangolo non cancellato e della shell
       if ( m_vTria[i].nIdVert[0] != SVT_DEL  &&  m_vTria[i].nShell == nShell) {
@@ -3989,10 +3999,12 @@ SurfTriMesh::IsShellClosed( int nShell) const
             bClosed = false ;
             break ;  
          }
+         else
+            ++ nTriaCnt ;
       }
    }
   // restituisco il risultato
-   return bClosed ;
+   return ( bClosed  &&  nTriaCnt >= 4) ;
 }
 
 //----------------------------------------------------------------------------