i8 DISCOVERY REPORTS 



theless, the data fall into two well-defined groups, one for S. thompsoni with a mean value of 177-7 ±3*4 

 for the aggregate and 93-2 ±2-8 for the solitary, and the other for S.gerlachei, with means of 128-8 ±3-2 

 and 48-8 ± 1-8 for aggregate and solitary, respectively. The difference between the species is even more 

 obvious from Text-fig. 5 based on the data of Table 4 which gives the percentage frequency of 

 occurrence of the various muscle fibres for the two species, and, for comparison, the data for S.fusi- 

 formis. Text-fig. 5 shows that in the solitary form S. thompsoni and S. gerlachei are quite distinct. 



(a) 



300. 



f2 200 



O 



a: 



100 



O 



I50 



10 



20 30 40 



BODY LENGTH (mms) 



50 



60 



> 



(b) : 



O 



10 



20 



30 



40 



120 



50 60 70 80 90 100 110 

 BODY LENGTH (mms) 



Text-fig. 3. Variation in the number of muscle fibres in S. fusiformis and S. aspera; (a) the aggregate form 

 (M. I-VI, see text) and (b) the solitary form (M. IV), plotted against body length. 



Although there is overlap between the data for the solitary forms of S. gerlachei and S. fusiformis there 

 can be little confusion between them when other features (such as the character of the external test) 

 are considered and particularly since they occur, as will be shown later, at the opposite ends of the 

 range of distribution of this group of four species (see Text-fig. 6). 



The data for the aggregate forms of S. thompsoni and S. gerlachei in Text-figs. 4a and 5 show that 

 the diflFerence between them is not so marked as it is in the solitary form. Nevertheless, two distinct 

 groups can be seen with means of 177-7 ^^'^ 128-8 for S. thompsoni and S. gerlachei, respectively, 

 (Table 3). It will be noted both from Table 3 and Text-fig. 5 that S. fusiformis with a mean of 

 51-8 fibres for M. I-VI cannot be confused in its aggregate form with either 5. thompsoni, S. gerlachei 

 or S. aspera. 



