ARBACIAD^. 65 



careous branches on each side. The shape of the blade differs greatly in the 

 different genera, and even in different species. 



The triphyllous pedicellaria; (Pi. 47, fig. o) are always uncommon, often 

 rare, and frequently wanting. When present, they are usually to be found 

 on the abactinal surface near the boundaries of the interambulacra, and rarely 

 on the genital plates. They are always small, provided with a long neck, 

 and intergrade almost completely with the tridentate. The head is only 

 .18-.40 mm. in length, but the stalk is commonly eight to ten times as long. 

 The valves (PL 46, fig. 16) are provided with a small articular loop, but these 

 do not overlap, and the three valves of a head are all alike. The apophysis 

 does not continue into the blade, nor is any calcareous network developed 

 there. No "cover plate," so evident in the triphyllous pedicellarioe of the 

 Aspidodiadematidge and EchinothuridtB, is ever present. 



Sphseridia (Pis. 47, figs. 10, 15-17 ; 48, fig. 19 ; 49, figs. 1, 9, 15, S6) are 

 present in the ambulacra of all the Arbaciadoe, at least at the peristome. 

 They show considerable diversity in size (.20-.50 mm. in diameter) and form, 

 and apparently vary more or less with the age of the individual (compare 

 figs. 15 and 16, PI. 47). No reliance can be placed on their form as a specific 

 character. They may be nearly or quite globular, and occasionally they 

 are longer than wide, but as a rule they are much wider than long. .They 

 are borne in an upright position on a very short stalk which is jointed to a 

 small knob or projection of the test. They may be placed on the surface of 

 the test with little or no concavity back of tliem(Podocidaris, Dialithocidaris, 

 Habrocidaris), or they may be more or less deeply sunken in pits in the test 

 (Arbacia, Coelopleurus). In Coelopleurus there are 6-12 sphiBridia in each 

 ambulacrum, arranged vertically, so that there is a pit at the inner angle of 

 each of the lowermost ambulacral plates (PI. 53, fig. 1). In all the other 

 genera a single sphaeridium is present in each ambulacrum close to the 

 peristomal margin. 



*The pedicels of the actinal surface are always provided with the usual 

 calcareous terminal rosette and its accompanying supporting plates (PI. 

 48, figs. 7, 8), but on the abactinal surface of the test these are commonly 

 wanting. In some species no other calcareous deposits appear to be present 

 in the pedicels, but usually some sort of supporting rods are found (Pis. 48, 

 figs. 9, 17, 21 ; 49, figs. 3, 11, 20, 27). These may be simple, rough, irregu- 

 lar, slightly curved, non-perforated rods (Coelopleurus), or straight, smooth 

 rods, expanded and perforated at the middle (Podocidaris, Habrocidaris, 



