236 POTAMILLA RENIFORMIS. 



bristle-tufts, perhaps, being more conspicuous than those which followed. The ventral 

 groove turned to the right behind the twenty-fifth scute. The two kinds of bristles, viz., 

 those with winged, tapered tips and those with broad paddle-tips, are present throughout 

 the series, though best developed in the first eight. The hooks and their attendant short 

 penniform or beaked bristles occur throughout the entire anterior region. The branchiae 

 have the usual structure and their ocelli remain in the preparation. This example 

 demonstrates the wide range of segments in the anterior region of Sabellids, as, indeed, 

 various figures of previous authors show ; thus Chenu 1 gives one with eleven pairs of 

 thoracic bristles. 



The setigerous processes are ranged along the lateral regions from the second 

 segment backward, a differentiation occurring anteriorly by the inflection of the groove 

 which often passes behind the tenth bristle-tuft to the mid- ventral line, though in others 

 it is further back. Bach of the setigerous processes anteriorly has dorsally three longer 

 bristles with straight shafts, tips bent backward and moderate wings (Plate CXXVIII, 

 fig. 2). The edges of the wings appear to be minutely serrated. Following these is a 

 double series of comparatively stout bristles with short and broad wings, making a 

 spatulate tip with a filament in the centre (Plate CXXVIII, figs. 2 a and 2 b). These 

 bristles also have a dorsal curve, the filament trending in that direction, so that they 

 would brush an opposing structure with the convex surface. From the nature of the parts 

 the shafts are somewhat abruptly tapered at the tip. Some of the bristles have modified 

 tips so that they resemble a short and broad knife-blade, as in certain forms in Chsetopt&ms, 

 the shaft not being continued along the centre as in the ordinary winged types. 



In the posterior segments the bristles alter, being shorter, fewer in number, and with 

 modified (geniculate) tips (Plate CXXVIII, fig. 2 c) which have moderately wide wings 

 at the base, but they soon diminish, and the long central tapering tip projects far beyond 

 them, thus performing the functions of the simple bristles of this region in other forms. 



The anterior rows of hooks are below the setigerous processes, and consist of a long 

 series of the avicular forms (Plate CXXVIII, fig. 2 e) with serrated crowns sloping to the 

 sharp main fang, a rather long, slightly striated neck with straight sides, the anterior 

 outline curving forward into the rounded prow and the posterior into the well-marked 

 basal process. Accompanying each is a bristle (Plate CXXVIII, fig. 2d), the shaft of 

 which has a curvature toward the distal end, and the tip has a region with short wings 

 so modified as to resemble a hook with a long shaft and a main fang. 2 Two forms of 

 accompanying bristles thus are present in this species, viz., those with broadly spatulate 

 tips, and those with a slightly enlarged posterior crown and a beak-like point anteriorly 

 nearly at right angles to the shaft. In a small example from Perelle Bay the latter 

 was large and with distinct wings (as in fig. 2 d). The book has a larger space between 

 the main fang and the prow than in Sabella penicillus. 



The hooks behind the foregoing region are above the setigerous processes, and they 

 become fewer and fewer as well as smaller, and with a longer base in their progress 

 toward the tail (Plate CXXVIII, figs. 2/, 2/ 1 ). 



1 ' Illust. Conch./ ll e livr., pi. vii, fig. 6. 



2 So far as can be seen at present certain specimens do not have the very broad paddle-like 

 bristles, though their hooks cannot be differentiated from those which have these broad bristles. 



