12 DISCOVERY REPORTS 



Ten specimens of the aggregate form taken in a i m. net towed obliquely (NiooB) from lOO m. 

 to the surface at Discovery station 2087, 44° 22-1' S., 23° 31' E. on 17 November 1937. 



Registration NUMBER. British Museum (Nat. Hist.), i960. 10.18. 8. 



Solitary Form (PI. Ila and c) 

 (Based on the examination of ninety-two specimens.) 

 External appearance. Elongate. Similar to S. aspera but more heavily serrated. 

 Test (PI. lie). Heavily serrated, firm, with thickened areas forming pronounced serrated ridges. 

 Number and disposition of lateral and ventral ridges is similar to S. aspera but dorsal surface is 

 more complex owing to a number of less pronounced ridges which I shall call secondary ridges to 

 distinguish them from the more obvious principal ridges. Although not always immediately obvious 

 the secondary ridges become particularly evident when stained with methylene blue (see p. 5). 

 The principal ridges are as follows : 



(a) inner and outer dorsal limiting ridges, 



(b) a dorsal-lateral ridge, 



(c) a ventro-lateral ridge, 



(d) posteriorly a single keel-like mid-ventral ridge which divides to form left and right branches or 

 ventro-limiting ridges. 



The secondary ridges are as follows: 



(e) a medio-dorsal ridge extending for a greater part of the length of the body, 



(/) a secondary dorsal-limiting ridge which runs between and parallel to the inner and outer dorsal- 

 limiting ridges, 



(g) a secondary dorso-lateral ridge which runs inside and parallel to the main dorso-lateral ridge, 



(h) connecting the inner and outer branches of the dorsal-limiting ridges is a short transverse ridge 

 at about the level of M. VHI. 



I shall refer to this arrangement of serrated ridges as ' complex '. 



Body muscles. Nine broad body muscles similar in their disposition to S. fusiformis. M. I-HI 

 fuse medio-dorsally. M. IV-VH are parallel. M. VHI-IX fuse medio-dorsally. 



Muscle fibres. Mean number 93-2 ±2-8 for M. IV, based on 92 observations. 



Alimentary canal. Compact, oval nucleus. 



Stolon. Similar to 5. aspera but it extends anteriorly much further before doubling back on itself. 



Length. Up to 120 mm. 



Aggregate Form (PI. lib) 



(Based on the examination of one hundred and twenty-six specimens.) 



External appearance. Body barrel shaped with conical anterior and posterior protuberances of 



the test. Great variation in body form. The anterior and posterior processes may be long, giving a 



typical ' fusiform ' shape, or shortened. 



Test. Serrated, firm. From the anterior protuberance a dorsal serrated ridge runs posteriorly to 



form the right dorsal-lateral edge of the test. At about the level of the atrial orifice this serrated ridge 



divides to form a right ventral-lateral ridge which reconnects with the dorsal-lateral ridge at about the 



level of the nucleus. On the left side of the test there are left dorsal and ventral-lateral ridges which 



originate in the posterior protuberance. In well-serrated animals the disposition of these ridges is such 



as to give, as Herdman (1888, p. 76) remarked, an 'angular or somewhat prismatic appearance' to 



the test. 



Body muscles. Six broad body muscles similar to S. fusiformis. M. I-IV and M. V-VI fused to 



form two dorsal groups. M. IV-V converge and join laterally. 



