THE iAMILT ARBACIAD^. 55 



On the plates at the ambitus and below, these broad promi- 

 nences become broken up into rounded knobs, very like the 

 structure in A. stellata, although they do not appear to form 

 more than two series, and generally one or more of the broader 

 " uniserial " prominences remain. 



The plates of the apical system are united together by a 

 dowelling of numerous small semiglobular knobs, less regular in 

 their disposition than any of the foregoing; the basal (genital) 

 plates bearing only knobs, whilst the radial (ocular) plates bear 

 pits only. 



In Tetrapygus (Arhacia) nigra the sutural dovetailing is very 

 remarkable, and highly developed. On the seventh plate from 

 the apex, it consists of a single series of large, broad, subtrian- 

 gular, plate-like prominences arranged transversely on the sur- 

 face of the facet, and slightly inclined aborally in relation to its 

 plane (see fig. 9). As the plates approach the ambitus, the plate- 

 like prominences gradually become broken up into rounded 

 elongate prominences, one half of the plate-like form remaining, 

 whilst the other is formed as it were into the smaller, but equally 

 long, prominences. Below the ambitus, on the actinal surface, 

 the breaking up is still further carried out, and we find a number 

 of small granule-like knobs arranged in wavy lines across the 

 plate, amongst which some apparent confluence is visible. 



In all these cases, the aboral facet of the adjacent plate in the 

 neighbouring column of th.e interradium to which the knobbed 

 facet is apposed, is furnished with exactly correspondent depres- 

 sions or pits, into which the prominences fit. 



In the species we have been discussing, there is a similar 

 system of dowelling, but less developed, at the junction of the 

 ambulacral plates and the interradial plates, liere present in the 

 form of small uniform rounded granule-like knobs ; and the 

 knobs are all borne on the ambulacral plates, whilst the adjacent 

 end of the interradial plates bears only pits. 



There is also a similar system of dowelling in the vertical or 

 median zigzag suture of the ambulacral area, the knobs being 

 borne on the aboral facet of the plate, whilst the pits are borne 

 on the adoral facet, the reverse of what takes place in the median 

 interradial suture. The dowelling in the median ambulacral 

 suture is comparatively feebly developed, and resembles that 

 between the ambulacral and the interradial plates, having, in some 

 instances, at first sight little more than a " carious " appearance. 



