14 DISCOVERY REPORTS 



Muscle-fibre number. 127 fibres for the sum of M. I-VI counted along the right side. 



Embryo. Single oozoid present. 



Paratypes. Five specimens of the solitary form taken in a i m. net (N lOoB) which fished between 

 440 and 190 m. at Discovery station 1257, 67° 52-4' S., 129° 27-5' W. on 11 January 1934. 



Registration number. British Museum (Nat. Hist.), i960. 10. 18.3. 



Ten specimens of the aggregate form taken in a i m. net towed obliquely (NiooB) from 109 m. 

 to the surface at Discovery station 1296, 69° o6-6' S., 131° 42-6' W. on i March 1934. 



Registration number. British Museum (Nat. Hist.), i960. 10. 18.4. 



Solitary Form (Text-fig. zd) 

 (Based on the examination of sixty-seven specimens) 



External appearance. Elongate. Similar to S. thompsoni. 



Test. Heavily serrated, firm. Pattern and number of serrated ridges identical with those in S. thomp- 

 soni, i.e. 'complex'. 



Body muscles. Nine thin body muscles. M. I-HI converge and join dorsally. M. IV-VH 

 parallel. M. VHI-IX converge and join dorsally. Area of contact of the muscles that join dorsally is 

 much smaller than in S. thompsoni. 



Muscle fibres. Mean number 48-8 ±1 -8 for M. IV, based on 67 observations. 



Alimentary canal. Compact, oval nucleus. 



Stolon. Similar to S. thompsoni. 



Length. Up to 75 mm. 



Aggregate Form (Text-fig. 2b) 



(Based on the examination of fifty specimens) 



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

 the test. Similar to S. thompsoni. 



Test. Serrated. Disposition of the serrated ridges is the same as in S. thompsoni. 



Body muscles. Six thin body muscles. M. I-IV and M. V-VI fuse dorsally to form two muscle 

 groups. M. IV-V converge and join laterally. 



Muscle fibres. Mean number 128-8 ±3-2 for the total of M. I-VI, based on 50 specimens. 



Table i . A summary of the features characterizing the four species of the 'fusiformis ' group 



J Ratio of body length: muscle width, see p. 23. 



