ECHINID^. 183 



Family 4— ECHINID^. 



This extensive natural family comprehends many genera of living and fossil urchins : 

 some of which are large and globular ; others are of moderate size, or small, hemispherical, 

 or depressed : in general the test is thin, and each column is composed of a considerable 

 number of plates. 



The ambulacral areas are about one third the width of the inter-ambulacral ; they have 

 two, four, or more rows of tubercles developed on their surface, which are often nearly 

 as large as those of the inter-ambulacral areas. 



The poriferous zones present considerable diversity in the number and arrangement 

 of the pores : in one section they are in single pairs ; in a second they form double rows ; 

 in a third they are in triple oblique pairs ; and in a fourth the wide poriferous zones have 

 the pores disposed in three vertical rows. 



The inter-ambulacral areas are more or less wide, and their large pentagonal plates 

 are four times as long as they are broad \ they are sometimes perforated at the 

 angles, as in Mespilia and Microcyplms ; or they have depressions iu the line of the 

 sutures, as in Temnechimis and Opecldnus ; the surface is sometimes sculptured with 

 irregular figures in relief, as in Glypticus, or finely and microscopically plaited as in 

 Codiopsis ; for the most part the plates have numerous tubercles developed on their surface. 



The tubercles are in general small, and nearly of the same size in the ambulacral and 

 inter-ambulacral areas ; their bosses have smooth summits, and they are always 

 imperforate ; there are often several rows on the same horizontal line. 



The spines are always short and subulate, and their surface is sculptured with fine 

 longitudinal lines. 



The mouth opening is sometimes small, and sometimes very large ; the peristome is 

 often pentagonal, and feebly indented ; or it is deeply incised, and divided by notches into 

 unequal-sized lobes. 



The apical disc is small, and composed of five genital and five ocular plates; 

 the spongy madreporiform body is always prominent on the right antero-lateral genital 

 plate. 



The large and powerful jaws are composed of the same pieces as in the Cidarida ; 

 but the pyramids are excavated in their upper part, and the two branches are united by an 

 arc at the summit : the teeth are long and tricarinated. 



In the following table I have endeavoured to classify the genera, and to show 

 at one view the most striking characters of the different groups included in this 

 family : 



