366 DISCOVERY REPORTS 



came to the conclusion from his own investigations that most probably all the different 

 species of Atolla hitherto described are variants or contraction phases of two species 

 only. The nine species previously supposed to be distinct have thus successively been 

 reduced to two only: A. zvyvillei with smooth lappets and A. chiwi with lappets studded 

 with small papillae ("glass-beads" of Vanhoeffen) (cf. Bigelow, 1928, p. 505). 



The study of the rich Discovery material has confirmed Browne's opinion. In the 

 present material I have distinguished two species only: zvyvillei and chuni. All the 

 characters, apart from the presence or absence of papillae on the lappets, which have 

 hitherto been used for separating the different species (i.e. presence or absence of radial 

 furrows on the central disc, form and number of the same, breadth of septal notches, 

 diameter of central disc in relation to the diameter of the bell, number of tentacles, 

 absence or presence of a smooth annular zone between the ring furrow and the pedalia 

 and of septal areas subdividing the tentacular canals near their base) have been proved 

 to be so highly variable that they are of little or no systematic value. 



Of all these variable characters of Atolla the sculpturing of the central lens, with or 

 without radial furrows at the margin, has been found the most useful. Although the 

 present series shows intermediate forms between the bairdi type with smooth central 

 lens, the verrilli type with the central lens scored with narrow furrows, and the zvyvillei 

 type indented with broad radial furrows, I have, following the excellent studies of 

 Bigelow, divided the comprehensive species zvyvillei into three types or forms : zvyvillei 

 (typica), bairdi and verrilli. The Discovery Expeditions brought home altogether 228 

 Atolla, of which 185 belong to A. zvyvillei and 43 to ^4. chuni. 



Atolla wyvillei, Haeckel 



(Plate XV, fig. 4; Figs. 3-6) 



2. xi. 25. 6° 55' N, 15° 54' W, south of Cape Verde. 2 m. tow-net, 0-800 m. 



Three specimens of bairdi, 11, 13, 15 mm. diam., faded, Broch's pigment group I. 



St. 298. 29. viii. 27. 13° 01' 45" N, 21° 34' 45" W, south of Cape Verde. Young fish trawl, 900- 

 1200 (-0) m. 

 Four specimens of bairdi, 21-62 mm. in diam. ; one specimen of wyvillei (perhaps transition-stage 

 to verrilli), 17 mm. diam. All pale, the last the darkest, traces of pigment on coronal furrow, ring 

 muscle greenish yellow, gonads free of pigment, group III of Broch ?. 



28. X. 25. 13° 25' N, 18° 22' W, south of Cape Verde. 4J m. tow-net, 0-900 m. 



Twenty-four specimens, all bairdi, 30-65 mm. diam., with smooth disc, very feeble notches at the 

 margin of central disc, gonads well developed, stomach deeply pigmented, traces of pigment on and 

 between pedalia and in central furrow, ring muscle without pigment, Broch's groups III and IV. 



St. 700. 18. V. 31. 20° 2ii' N, 22° 33I' W, south of Cape Verde. Young fish trawl, 2025-0 m. 



One specimen of bairdi, 48 mm. diam., central disc 34 mm. diam., very beautiful, with feeble 

 notches on the margin of central disc reddish brown, circular muscle greenish, not pigmented, 

 gonads well developed. Broch's pigment group IV. 



St. 281. 12. viii. 27. 00° 46' S, 5° 49' 15" E, Gulf of Guinea. Young fish trawl, 850-950 (-0) m. 



Three specimens of zvyvillei {verrilli), 25-27 mm. diam., pale, stomach with gonads without 

 pigment, traces of it on exumbrella, ring muscle without pigment, Broch's group III. 



