SEXUAL CHARACTERS—ELASMOBRANCH FISHES 257 
a longitudinal dorsal muscle-band external to the whole siphon 
and afterward to be described in Raia (fig. 11). These two 
longitudinal groups may have a coordinating function. 
The parasiphons are vestigial in 8. canicula. Out of one 
hundred specimens of this fish chosen at random indications of 
B. c. iD: 
0) a 
Figure 7 
Fig. 6 Scyllium canicula, an enlarged view (from fig. 3) of the siphon in 
transverse section; S., siphon; m., muscle; Ep:., stratified epithelium; b.v., blood- 
vessels. 
Fig. 7 Scyllium canicula, a diagram of the relative proportions of vestigial 
parasiphons, as explained in the text. 
parasiphons were recognizable in only thirty. In figure 7, 
diagram A represents the siphon (S) and a parasiphon (P) in 
dotted outline drawn to the same scale as it would appear if 
developed to the same extent as in 8. catulus. Diagram B 
shows its vestigial development as exhibited by 3 per cent of the 
selected individuals; C as in 7 per cent; D as in 20 per cent; the 
remainder were without trace of parasiphons. 
