THE EAMTLT AEBACIAD^. 57 



Fig. 3. Ccelopleurus eguis. A compound ambulacral plate from the region 

 of the ambitus, a, the lai-ge primary poriferous plate ; V, the aboral 

 demi-plate ; b", the adoral demi-plate. Magnified. 



4.- Cmlo'pleurus sindensis. Eadial plate and neighbouring abactinal region 

 of the ambulacrum. Magnified. 



5. Eadial plate and abactinal portion of the ambulacrum, to show the 

 manner of the formation of the compound ambulacral plates. Mag- 

 nified. 

 *6. An incipient compound ambulacral plate, formed of the 6th, 7th, and 

 8th primary poriferous plates. Magnified. 



7. Ccelopleurus Maillardi. An ambulacral plate near the peristome, 



showing the broad plates and a spheridial pit. Magnified. 



8. Portion of the ambulacrum IV., seen from within. Magnified. 

 *9. One of the large tubercles. Magnified. 



10. Arrangement of the peripodia at the peristome. Magnified. 



11. The end of an interradial plate, in the median suture, showing the 



knobs and pits. Magnified. 



12. The corresponding facets of the two adjacent plates in the neigh- 



bouring column, furnished with reciprocal pits and knobs, into which 

 those of the plate shown in fig. 11 fit. 



Plate II. 



Fig. 1. Arbacia pustulosa. Compound ambulacral plates from the region of 

 the ambitus, a, the large primary poriferous plate ; b', the aboral 

 demi-plate ; b", the adoral demi-plate. Magnified. 



2. Arbacia punctulata. A compound ambulacral plate from the region 



of the ambitus. Magnified. The same lettering as the aboye. 



3. Arbacia stellata. Eadial plate and abactinal portion of the ambu- 



lacrum IV., to show the manner of the formation of the compound 

 ambulacral plates. Magnified. 



4. Portion of an ambulacrum and adjacent interradial area near the 



peristome, to show the arrangement of the peripodia. Magnified. 



5. The same, showing the constitution of the compound ambulacral 



plates. Magnified. 



6. Tetrafygus {Arbacia) nigra. A compound ambulacral plate. «', the 

 primary poriferous plate ; b\ 5", 5'", the demi-plates. Magnified. 



7. Arbacia stellata. The end of the 7th interradial plate from the apex 



in the median suture. The adoral facets with knobs only, the aboral 

 with pits. Magnified. 



8. Arbacia punctulata. The end of the 7th interradial plate from the 



apex in the median suture. Magnified. 



9. Tetrapygus {Arbacia) nigra. The end of the 7th interradial plate 



from the apex in the median suture. Magnified. 



* In the lettering of Plate I. the numbers to figs. 6 and 9 have been inad- 

 vertently misplaced, and should be transposed. 



LIUIf. JOTJRN. — ZOOLO&T, VOL, XIX. 5 



