1903.] 



MONSTROSITIES IX FISHES. 21 



surrounding the fused adjacent Meckelian and hyoid bars in 

 Class I. (p. 10) are destitute of nerves. 



Development of parts may take place without the presence of 

 nerves of general sensation. Thus in Class I. (a) no trace exists of 

 adjacent trigeminal nerve-fibres, yet the inner sides of the snout 

 and head are well developed wherever they have sufficient 

 space. 



On the whole, the anatomy of the series of monstrosities which 

 has been described in this paper is remarkably ^symmetrical, both 

 twins contributing equally to the sum of structures in the 

 transitional region, in which, moreover, the law that union takes 

 place between homologous structui-es aiway holds good. 



EXPLANATION OP THE PLATES. 



{Reconstruction Diagrams^ 



Plate I. 



Fig. 1. Cranial and hyomandihular skeleton of a normal Trout embryo, seen from 

 above, after removal of the roof -cartilages by a section supposed to pass 

 horizontallj' through the nasal and periotic cartilages (pp. 5, 6). 



a, olfactorj^ pit and cartilage. 



h, trabecuhe cranii. 



c, parachordal cartilages. 



d, placed just in front of pituitarj' space. 



e, „ behind pituitary space. 



f, palato-quadrate bars. 



g, Meckel's cartilage. 

 h, hyoid bar. 



i, hj'omandibular. 



Ic, placed in floor ot periotic capsule. 



m, pituitary space with external recti muscles passing through it. 

 n, uotochord embedded in the parachordals. 

 Fig. 2. Roof-cartilages of skull of a normal Trout embryo, seen from above (p. 6). 



(For lettering see under fig. 6.) 

 Fig. 3. Cranial and hyomandihular skeleton of a double-monster embryo, seen from 

 above as in fig. 1. The duplicity affects the anterior cranial structures only 

 (pp. 7, 8). Lettering as above ; see also under fig. 5. 

 Fig. 4. Roof-cartilages of skull of the double monster illustrated in fig. 3. For 



lettering see under fig. 6. 

 Fig. 5. Cranial and hyomandihular skeleton of a double monster exhibiting a 

 slightly greater degree of duplicity than that of fig. 3 (pp. 11, 12). 



In figs. 3 & 5, a, h, c, &c. represent the same structures as in fig. 1 ; 

 f, g', i', k' are the inner or adjacent (reduced) structures corresponding to 

 the fully-developed outer /, g, i, k {i. e. palato-quadrate, Meckelian, hyo- 

 mandihular, and periotic cartilages). 

 Fig. 6. Roof-cartilages of sktill of the double monster illustrated in fig. 5. 

 yi, f2, f'i. Anterior, middle and posterior fontanelles. 

 r, cartilage of roof of nasal pits, 

 s, supraorbital bars (.9', adjacent reduced pair). 

 t, cartilage of roof of periotic capsule. 

 ■w, parachordal cartilage. 

 X, y, tegminal cartilages above third ventricle and cerebellum respectively. 

 Fig. 7. Vertebral cartilages in normal Trout embrj^o (p. 6). 



Fig. 8. „ „ „ any double monstrosit}' near the transitional region. 



Spc. Spinal cord. 

 J!Vc7i. Notochord. 

 Na. Neural arch cartilage (A"a', adjacent reduced neural arch 



cartilage). 

 Ka. Hasmal arch cartilage {Ha', adjacent reduced haemal arch 

 cai'tilage). 



