Fig. 4. Transverse section through the ventral part of the trunk of an embryo _Scyllium_ of stage P, in the region of the pectoral fins, to shew how the fins are attached to the body, magnified 18 diameters. _br._ cartilaginous fin-ray. _bp._ basipterygium. _m._ muscle of fin. _mp._ muscle-plate. Fig. 5. Transverse section through the ventral part of the trunk of an embryo _Scyllium_ of stage P, in the region of the pelvic fin, on the same scale as fig. 4. _bp._ basipterygium. _br._ cartilaginous fin-rays. _m._ muscle of the fins. _mp._ muscle-plate. Fig. 6. Pectoral fin of an embryo of _Scyllium canicula_, of a stage between O and P, in longitudinal and horizontal section (the skeleton of the fin was still in the condition of embryonic cartilage), magnified 36 diameters. _bp._ basipterygium (eventual metapterygium). _fr._ cartilaginous fin-rays. _pg._ pectoral girdle in transverse section. _fo._ foramen in pectoral girdle. _pe._ epithelium of peritoneal cavity. Fig. 7. Transverse section through the pectoral fin of a _Scyllium_ embryo of stage P, magnified 50 diameters. _bp._ basipterygium. _br._ cartilaginous fin-ray. _m._ muscle. _hf._ horny fibres. Fig. 8. Pectoral fin of an embryo of _Scyllium stellare_, magnified 16 diameters. _mp._ metapterygium (basipterygium of earlier stage). _me.p._ rudiment of future pro- and mesopterygium. _sc._ cut surface of a scapular process. _cr._ coracoid process. _fr._ foramen. _hf._ horny fibres. Fig. 9. Skeleton of the pectoral fin and part of pectoral girdle of a nearly ripe embryo of _Scyllium stellare_, magnified 10 diameters. _mp._ metapterygium. _mes._ mesopterygium. _pp._ propterygium. _cr._ coracoid process. EXPLANATION OF PLATES 34-42. (XXII. p. 738.) LIST OF REFERENCE LETTERS.