54 PROP. p. M. DUNCAN AND MB. "W. P. SLADEN ON 



facet. These structures recall, more or less vividly, the system 

 of dowelliug by knobs and sockets, previously described by one of 

 us in Temnopleuridae * ; in the case of Arbacia, however, they 

 are confined to the -vertical sutures entirely, no trace whatever 

 of their existence being found on the horizontal sutures which 

 separate the successive plates in a column : or in any case only a 

 slight granular surface, presenting, on separation, a faintly 

 carious appearance. 



The special form of the knobs and pits varies in the different 

 species that we have examined, and would appear to constitute 

 a good secondary specific character ; it also varies a little, but in 

 a definite manner, according to the position of the plate upon 

 which it occurs — in other words, the dowelling on the abactinal 

 surface is slightly different from that on the actiual surface. 

 This structure appears to exhibit its maximum development on 

 the abactinal surface about midway between the ambitus and 

 the apex. 



On examining the seventh interradial plate (counting from 

 the apex) of A. stellata, a number of small round semiglobular 

 prominences will be seen forming more or less regularly three 

 lineal series, which occupy about half the area of the facet. The 

 eminences and pits on the respective facets are confined to a well- 

 defined area, between which and the edges of the facet that cor- 

 respond to the internal and external surfaces of the plate respec- 

 tively a straight smooth margin intervenes ; the knobbed and 

 pitted area being rather nearer to the internal than the external 

 surface of the plate (see fig. 7). Nearer the ambitus the granular 

 knobs are more numerous and less regular in tbeir disposition. 



In A. punctulata, on the seventh plate from the apex, instead 

 of the semiglobular rounded knobs found in A. stellata, we have 

 a series of larger and more elongate prominences, either oblique 

 in position or subparallel with the lateral sides of the facet, the 

 individual prominences becoming narrower and suboval in con- 

 tour as they approach the end of the facet adjacent to the aboral 

 facet of their own plate (see fig. 8). Eoughly speaking, they 

 may be said to form only a single series (though there is a slight 

 tendency to double in the part where the smaller-sized knobs 

 just mentioned occur); and the area they occupy is narrower 

 than the area occupied by the knobs in A. stellata. 



* P. Martin Duncan, Linn. Soc. Journ., Zool., vol. xvi. pp. 343-358. 



