PLATE XVIII 



Fir.. 64.— Ventral ends of the Meckelian, hyoid, and 

 succeeding visceral arch cartilages of a double monster 

 belonging to Class I. For general references see under 

 PI. I. fig. 4. Lettering as in the succeeding figure. 



Fig. 65.— Ventral ends of the Meckelian, hyoid, and 

 succeeding visceral arch cartilages of a double monster 

 belonging to Class II. For general references see under 

 PL II. fig. 6. 



Meek. Outer Meckelian bars. 



Meek'. Inner or adjacent Meckelian bars united and greatly 



reduced. 



C,H. Glossohyal. 



BH. Hypohyal. 



LH. Ceratohyal. 



BH'. Inner or adjacent hypolvyals paitly united: 

 LH'. ,, ,, ceratobyals united. 



Ill, IV, V, VI, VII. The branchial cartilages. 



Cop. Copular pieces, the anterior one being bifid. 



Fig. 66. — Transverse section of skeleton of composite 

 pectoral fin from a double monster belonging to Class IV., 

 in which union of the twin bodies took place just behind 

 the pectoral fins. Description on p. 19. 



Scap.. Cor. Scapular and coracoid parts respectively of the coraco- 

 scapular bars. The ventral ends of the coracoids 

 are united. 

 ' /.. The limb-cartilages united for the greater part of their 

 length. 

 R. The rays (not cartilaginous). 



FIG. 67. — Diagram illustrating the dorsal aorta and its 

 roots in the double monster belonging to Class I., from 

 which PI. XVIII. fig. 64 was taken. For references see 

 under PL I. fig. 4. 



Act. Aorta, the two limbs uniting further back. 

 II, I, III, IV. Aortic roots, i.e. branchial veins. 

 Car. Carotids. 



Aa. Artery to pseudobranch, a branch of the hyoid artery. 

 Psir. Pseudobranch. 

 A Char. Choroidal artery. 



A.Chor'. The arteries for the choroidal glands of the inner or 

 adjacent pair of eyes. They arise by a single stem 

 from the middle of a vessel connecting the aortic roots 

 on either side, and go to the choroidal glands without 

 passing through a pseudobranch. 



FlG. 68.— Diagram illustrating the ventral aorta, the 

 dorsal aorta, and the aortic roots in the double monster 

 belonging to Class II., from which )'!. XVII. fig. 60 was 

 taken. For references see under PI. II. fig. 6. Letter- 

 ing as in Fig. 67 with additional : 



I A. Ventral aorta and its branches 1, 2, 3. 4. 

 ' ar>. Inner or adjacent carotids, giving off Aa'. arteries to the 

 inner or adjacent pseudobranchs Psbr 1 . 



It will be seen that there are two sets of carotid and 

 afferent pseudobranch arteries, the inner sets being derived 

 directly from the ventral aorta. The ventral aorta arches 

 dorsally in the septum between the two mouths of the 

 monstrosity, reaches the base of the skull, and then 

 divides into two limbs which are continued backwards to 

 join the aortic-collecting roots on either side. 



Fir,. 69.— Outline of central cavity of brain of normal 

 trout embryo. Lettering as in Fig. 71. For references 

 see under PI. I. fig. 1. 



Fig. 70.— Outline of central cavity of brain of the double 

 monster (Class I.) illustrated in PI. XVII. fig. 58. Letter- 

 ing as in Fig. 71. For references see under PI. I. fig. 4. 



PlG. 7'— Outline of central cavity of brain in the double 

 monster (Class I.) illustrated in PI. XVII. fig. 60. For 

 references see under PI. II. fig. 6. 

 C.F.B. Cavity of the hemispher< 

 Cyd V. ,, 3rd ventricle. 



COL. ,, optic lobes. 



'- ,l - .. Her a tertio ad quartum ventriculum. 



C^thV. ,, 4th ventricle. 



Spc. ,. spinal cord. 



FIG. 72.— Outline of transverse section of central cavity 

 (in anterior part of optic lobe region of the brain illustrated 

 in Fig. 70 (r/.T-.). There are two infundibula and two sets 

 of hypoarial cavities, while the main optic lobe cavity is 

 single. 



' OL. Cavity of optic lobes. 



Inf. Infundibulum. 

 Hyp. Hypophys 

 Hya. One of the hypoarial cavitii 



