<?php
$cat = get_query_var('cat');

if(!get_category($cat)->category_parent) : //check if this is top-category

$categories=get_categories('child_of='.$cat);
if ($categories) {
 foreach($categories as $term) {

   echo '<h1><a href="' . get_category_link( $term->term_id ) . '" title="' . sprintf( __( "View %s" ), $term->title ) . '" ' . '>' . $term->name.'</a></h1>';
   echo '' . $term->description.'<br><br>';
 }
}
?>

<?php $args = array(
'category__in' => $this_cat,
'paged' => get_query_var('paged')
); //prepare to show only direct post of this category
query_posts( $args ); ?>

<? endif; // end of check ?>

<ol>
<?php while (have_posts()) : the_post(); ?>
<li><h3><a href="<?php the_permalink(); ?>">
<?php the_title(); ?></a></h3>
<div style="float:left; margin-bottom:10px;  padding-right:5px;" id="thumb">
<?php the_post_thumbnail('thumbnail'); ?>
</div>
<div id="postdescrip" style="margin-bottom:50px; ">
    <?php the_excerpt(); ?>
    </div>
    </li>
<?php endwhile; ?>
</ol>
<div class="navigation">
  <div class="alignleft"><?php previous_posts_link('&laquo; Previous') ?></div>
  <div class="alignright"><?php next_posts_link('More &raquo;') ?></div>
</div>