THE ECHINODERMATA 



267 



The mouth lies far forward (four-fifths of the whole 

 length) and faces forwards instead of downwards, as a 

 distinct lower lip (labrum} projects on its under side. 



In the apical disc the two posterior ocular plates are in 

 contact, the genital plates between them being crowded 

 out ; the madreporite is large and extends into the centre 

 of the disc, taking a flask-like shape and in contact with 



FlG. 77. MlCRASTER COR-ANGUINUM, LESKE, EMSCHERIAN 

 (UPPER CHALK). 



a, Aboral view. (xJ.) 3, Oral view. (Xi-) c, Side view. (Xj.) 

 d, Apical disc. (X~4.) (After Wright.) Some of the interamb sutures 

 are indicated in a and If. A, anus; Car, carina ; Fa, sub-anal fasciole ; 

 Lab, labrum ; M, mouth ; Mad, madreporite ; PI, plastron. 



all the other plates except the left posterior ocular. 

 Otherwise the plates retain a fairly regular arrangement. 

 The ambs are composed, near the apical disc, of close- 

 set plates, very short and broad, with uniserial pore- 

 pairs. At about half-way towards the ambitus a sudden 

 change takes place : the plates become much larger and 

 squarer, and their pore-pairs become much less notice- 



