Next & Previous (Pagination in this case) Not Appearing

<?php
/*
Template Name: Portfolio
*/
?>

<?php get_header(); ?>

<div id="full_container">
<div id="portfolio_content">
<div id="portfolio_wrap">
<div id="content">

    <?php 
        $loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 2)); 
    ?>
    <?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
    <?php   
        $custom = get_post_custom($post->ID);
        $screenshot_url = $custom["screenshot_url"][0];
        $website_url = $custom["website_url"][0];
    ?>


<a href="https://wordpress.stackexchange.com/questions/30616/<?php the_permalink() ?>">

<span class="img">
<?php if ( has_post_thumbnail() ) { the_post_thumbnail( 'thmb-portfolio' ); } ?>

<span class="under">
<!-- Excerpt title -->
<span class="title"><?php the_title(); ?></span>

<!-- Excerpt description -->
<span class="desc">
    <?php my_excerpt('short'); ?>
</span>
</span>
</span>
</a>
<?php previous_posts_link('Previous'); ?> / <?php next_posts_link('Next') ?>

        <?php endwhile; ?>  

<?php if (function_exists("pagination")) {
    pagination($additional_loop->max_num_pages);
} ?>

</div>
</div>
</div>
</div>
</div>

<?php get_footer(); ?>
Related Posts:
