ON THE FAMILY AEBACIAD^. 25 



that eleventh group and their group of Cariiivora. It has been 

 often suggested that the Bears are the nearest existing allies of 

 the Seals. There is, however, little in the anatomy of the true 

 Seals to force conviction on us as to this matter ; but vphen we 

 turn to the brain of Otaria, we may, I think, therein read a very 

 striking confirmation of the suggestion. The Arctoid group, and 

 the Arctoid group alone of all Carnivora, possesses an " Ursine 

 lozenge," That striking and exceptional feature is plainly to be 

 seen (as herein pointed out) in the brain of Otaria. This cir- 

 cumstance, I venture to think, makes it very highly probable 

 that the ancestor of the Pinnipedia was an Arctoid animal not 

 very remote from the Ursine group of that extensive suborder. 



In conclusion I desire to express my thanks to the authorities 

 of the Eoyal College of Surgeons for the great facilities afforded 

 me in studying the subject, and for their kindness in permitting 

 drawings to be made from the specimens therein preserved. 



On the Pamily Arhaciadcs, Grray. — Part I. The Morphology of 

 the Test in the GTenera Ccelopleurus and Arhacia. By P. 

 Martin Duncan, F.E.S., V.P.L.S., and W. Percy Sladen, 

 P.L.S. 



[Eead 5th February, 1885.] 



(Plates I. & II.) 



Contents. 



I. Introduction. — II. Fossil Ccelopleuri, the ambulacra. — III. The 

 radial plates. — IV. The apical sutures. — V. The interradial plates. — VI. 

 Recent Ccelopleuri. — Ccelopleurvs Maillardi, the ambulacra. — VII. The 

 interradial plates and their sutures. — VIII. The radial plates. — IX. The 

 2iTa\)\x\SiCX& oi Arbacia stellata, A. pnnctulata, siudi A. pustulosa. — X. The 

 ambulacra of Arbacia nigra. — XI. The structure of the vertical sutures 

 of the interradia. — XII. The comparison between the genera. — XIII. 

 Description of Plates. 



I. Introduction. 



The Arbaciadae of Gray, or the Echinocidaridse of Desmoulins, 

 have been studied, more or less, by nearly every naturalist who 

 has investigated or classified the Echiuoidea. 



The embryology of species of the genus which gives the name 

 to the family has been studied by A. Agassiz, Selenka, Busch, 



