1905.] NATURAL SCIENCES OF PHILADELPHIA. 847 



lapped by the branchial ridge medially and more extensively by the 

 paleolar tubercles laterally. This region is biannulate. The third 

 somite is somewhat enlarged and tumid ventrally, the dorso-lateral 

 portions project as thickened prominences bearing the paleoli, while 

 the dorsal half is merely a low welt, strongly concave along its anterior 

 margin and partly covered by the branchial ridge. 



Somites IV and V are much shorter, together barely equaling III, and 

 coalesced dorsally to form the ridge upon which the branchia3 are sup- 

 ported. The branchiae form a group of four on each side so arranged 

 that three are attached in a transverse anterior row on the region corre- 

 sponding to somite IV and only one more posteriorly on V. Between 

 the branchial scars of the two sides is a smooth quadrate area nearly 

 twice as long as broad which is slightly inserted posteriorly into a 

 transversely elongated area which reaches across the entire distance 

 between the posterior pair of scars. A single branchia remains. This 

 is slightly flattened, especially toward the tip, s regularly tapered to 

 a slender end and its length about equals the greatest diameter of the 

 thorax. 



The next four or five somites increase slightly in both length and 

 diameter, and then decrease in diameter but remian of constant length 

 throughout the thoracic region. Each is divided into a dorsal and a 

 ventral half-ring by the prominent setigerous tubercles. The latter 

 have thick glandular walls and the intersegmental furrows are deep 

 and well defined. The dorsal region is thin-walled and smooth, and 

 the first five intersegmental furrows only are distinct. 



Abdominal segments taper gradually to the pygidium and are no- 

 where distinctly defined by furrows. Dorsally the walls are high- 

 arched, thin and smooth, ventrally they are thickened by a pair of 

 prominent longitudinal muscle ridges with a neural groove between. 

 Laterally between the parapodia of successive segments is a series of 

 rugous areas, while across the ventral surface between each pair of 

 parapodia a narrow glandular line frequently extends. 



The pygidium is provided with a pair of thick lips bounding the ver- 

 tical, slit-like anus and each bearing on the side an inconspicuous cirrus 

 much shorter than the diameter of the pygidium. 



Of the setigerous tubercles the pair bearing the paleoli are prominent 

 and much larger than any others and they encroach nnich upon the 

 lateral portions of the peristomium. The paleoli are about twenty 

 (18-21) in number in each group and are inserted with their long diam- 

 eters radial around an arc of nearly two-thirds of a circle. From this 

 base they spread forwards and laterad in a broad, scoop-like figure. 



