<div id="taxon-concept-search">
  <%= form_for @search_params, :url => admin_taxon_concepts_url, 
    :method => :get, :html => {:class => 'form-inline'} do |f| %>

    <div class="input-prepend">
      <span class="add-on"><i class="icon-search"></i></span>
      <%= f.text_field :scientific_name, :class => 'search-typeahead input-xxlarge', :autocomplete => :off %>
    </div>

    <%= f.fields_for :taxonomy do |ff| %>
      <%= ff.select :id,
        options_from_collection_for_select(
          @taxonomies, "id", "name",
          @search_params.taxonomy && @search_params.taxonomy[:id]
        ), :class => "input-xlarge"
      %>
    <% end %>
    <%= f.submit 'Search'%>

  <% end %>
</div>

<div id="taxon-concept-search">
  <%= form_for @search_params, :url => admin_taxon_concepts_url, 
    :method => :get, :html => {:class => 'form-inline'} do |f| %>

    <div class="input-prepend">
      <span class="add-on"><i class="icon-search"></i></span>
      <%= f.text_field :scientific_name, :class => 'search-typeahead input-xxlarge', :autocomplete => :off %>
    </div>

    <%= f.fields_for :taxonomy do |ff| %>
      <%= ff.select :id,
        options_from_collection_for_select(
          @taxonomies, "id", "name",
          @search_params.taxonomy && @search_params.taxonomy[:id]
        ), :class => "input-xlarge"
      %>
    <% end %>
    <%= f.submit 'Search', :class => "btn"%>

  <% end %>
</div>

Set search with btn class
Set search with btn class
