xviii CLYPEASTROIDEA — SPATANGOIDEA 549 



Sub- Order II. Euclypeastroidea. 



Amis on under snrtaee. 



Fam. 1. Fibularidae. — The " petals " are short and imperfect, 

 and the internal skeleton consists of two short outwardly- 

 directed septa in each interradius. To this family the only 

 British Clypeastroid, Echinocyamus pusillus, belongs. This 

 animal never exceeds an inch in length, and has an oval 

 outline. It inhabits shallow water, and is often found in the 

 same ground as Echinus miliaris, but like all Clypeastroids it 

 prefers a sandy bottom. 



Fam. 2. Echinanthidae or Clypeastridae. — " Petals " well 

 marked, internal skeleton consisting of isolated pillars. The 

 largest Cake -urchins belong to this family, which is found 

 chiefly in tropical waters. Gly f easier, the great Cake -urchin, 

 with a deeply sunken peristome, belongs to this family. 



Fam. 3. Laganidae. — Closely allied to the foregoing, but dis- 

 tinguished by the fact that the internal skeleton consists of walls 

 parallel to the edge of the test. {Laganmn, Araclmoides, FeronellaS) 



Fam. 4. Scutellidae. — This family includes about half tlie 

 genera, and is sharply distinguished from all the rest by 

 (1) the extremely flattened shape, (2) the indentation of the 

 outline in the anal interradius and often elsewhere, (3) the 

 branching of ambulacral furrows on the under surface. Echin- 

 arachnius, taken as the type in describing the anatomy of the 

 Cake -urchins, is the best -known genus. Others are Mellita, 

 with five perforations in the edge of the test ; and Eohda, with 

 the edge produced into a number of finger-like processes. 



Order III. Spatangoidea {Heart-urchins). 



As the type we tnay select Echinocardium cordatuni, which 

 occurs abundantly in the Clyde and on the west coast of Ireland. 

 The animal is found buried in sand at a depth of about 8-10 

 inches from the surface. At this depth it lies in a burrow, the 

 walls of which are kept from collapsing by tlie somewhat 

 broadened tips to the spines. This burrow communicates with 

 the surface by a narrow cylindrical opening similar to the 

 opening of the burrows made by the Clams and other bivalves. 

 A little practice, however, enables one to distinguish the burrow 

 of the Heart-urchin from these. 



