1881.] PROF. F. J. BELL ON THE ECHINOMETRID^. 417 



distinguished from the neoproctous ; and they completely retain the 

 primitive disposition of the primary pore-plates. 



In the neoproctous group some of the primary plates always lose 

 their primitive relations ; but iu the Arbaciadae and the Diadema- 

 tidse this does not always affect the plates above the ambitus, and the 

 poriferous zones are in both almost straight ; they are palceosticka, 

 as compared with the Echinidse and Echinometridse, which are 

 neosticha ; and while the Arbaciadaj present a Cidarid character in 

 the want of connexion between the auricles, the Diadematidse (as 

 represented by Biademd) present a curious ancestral character la 

 the possession of rudimentary internal gills (Ludwig). 



Coming now to the Echinometridse and the Echinidse, we are at 

 once struck by the fact that it is impossible to find any points of 

 difference between them which are nearly so great, or of such 

 clear systematic value as (1) the presence or absence of external gills, 

 (2) the connexion of the auricles, (3) the presence of a subanal plate, 

 or (4) the number of the perforated buccal plates. Both families are, 

 in other words, branchiate, decalepid, neoproctous, neostichous. 



These relations are exhibited in the following Table : — 



Table of the Groups of the Echinoidea regularia (seu 

 Desmosticha). 



No external gills. Auricular arch not complete and not radial. 

 Ambulacral and interambulacral plates continued on to buccal mem- 

 brane ; pores in straight rows, all the pore-plates primary and sub- 

 equal. Entobranchiata '. Fam. 1. Cidaridce. 



External gills, auricles radial ; interambulacral plates not con- 

 tinued on to the buccal membrane. Ectobranchiata. 



Series a (Palaeoproctous). 

 Large suranal plate persistent in apical area. Fam. 2. Salenidce. 



Series /3 (Neoproctous). 

 Anal plates all secondary. « 



Subseries i. (polylepid). 

 More than one pair of ambulacral plates carried on to the buccal 

 membrane from each area. Fam. 3. Echinothuridce. 



Subseries ii. (decalepid). 

 Only five pairs of ambulacral plates on the buccal membrane. 



A. Auricular arch not complete. Fam. 4. Arbaciadce. 



B. Auricular arch complete ; rudimentary internal gill still re- 

 tained (Diadema). Fam. 5. Diadematidce. 



C. Auricular arch complete ; no rudimentary gill. 



Fam. 6. Echinid<B. 



1 The term Entobrfincliiata appears to me to be preferable to Abraachiata ; 

 and I cousequeutly use Ectobranchiata in place of Branchiata (Ludwig). 



