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Show 1881.] PROF. F. J. BELL ON THE ECHINOMETRIDsE. 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 in the Arbaciadse 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 palceosticha, as compared with the Echinidse and Echinometridse, which are neosticha ; and while the Arbaciadse present a Cidarid character in the want of connexion between the auricles, the Diadematidse (as represented by Diadema) present a curious ancestral character in 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 membrane ; pores in straight rows, all the pore-plates primary and sub-equal. E N T O B R A N C H I A T A 1 . Fam. 1. Cidarida?. External gills, auricles radial ; interambulacral plates not continued on to the buccal membrane. E C T O B R A N C H I A T A. Series a (Palseoproctous). Large suranal plate persistent in apical area. Fam. 2. Salenida?. 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. Echinothurida?. Subseries ii. (decalepid). Only five pairs of ambulacral plates on the buccal membrane. A. Auricular arch not complete. Fam. 4. ArBaciada?. B. Auricular arch complete; rudimentary internal gill still retained {Diadema). Fam. 5. Diadematida?. C. Auricular arch complete ; no rudimentary gill. Fam. 6. Echinida?. 1 The term Entobranchiata appears to me to be preferable to Abranchiata ; and I consequently use Ectobranchiata in place of Branchiata (Ludwig). |