760 EXPLANATION OF PLATES. 



CuviER, 6 the sphenoid bone, 1 1 the great ala of the sphenoid, 

 12 the OS mastoideum ; of the suspensory bones of the lower 

 jaw, the epi-tympanicum 23, the os-tympanicum 27, the malar 

 bone 26 (Jiypotympanic Owen). Behind these is placed the 

 gill-cover, namely: 30 prceoperculum, 28 operculum, 32 suh- 

 operculum, 33 interoperculum. Of the bones of the face, 19 is 

 the first of the sub- orbital bones, 17 the intermaxillary bone, 

 18 the superior maxillary bone, 18' accessory bony plate, 22 

 (omitted in fig. it is the bone seen in shadow behind the upper 

 part of the superior maxillary) the palate bone, 34 the dental 

 portion"of lower jaw. — 37, 38 The two flat bony pieces {cornua 

 of the hyoid bone), to which the rays of the brancliiostegous 

 membrane 43 (here fourteen in number) are attached; 40 

 bone connecting the cornua with the body of the hyoid bone; 

 41 glosso hyale; 42 the unpaired bony plate which runs back- 

 wards from the tongue-bone. 



Fig. 5. Scale of a pike, magnified about 7 diameters, compare p. 46. 

 The posterior margin a of the free edge of the scale is smooth. 

 Consequently the scale belongs to the cycloidea of Agassiz; 

 compare p. 81. 



Fig. 6. Scale of a perch, similarly magnified, the posterior margin a 

 is provided with numerous spines. This figure illustrates the 

 ctenoides of Agassiz, compare p. 1 30. 



Plate II. Fishes, continued. 



Fig. 1. Gills and heart of a perch in the natural position, seen from 

 the left side and somewhat from below. 



The gill-cover and gill-rays have been removed; a, the 

 heart; h, the four branchial arches of the left side, from 

 without; c, the inferior margins of the gills of the right side 

 seen from within. Compare p. 33. 



Fig. 2. Stomach and pyloric appendages of the cod, Gadus morrhua. 

 The duodenum and posterior extremity of the stomach are 

 laid open. In the former are seen four openings of the coecal 

 appendages. Compare pp. 25 — 27. 



Fig. 3. Hea.d of Peti'oniyzon Jluviatilis seen fi-om above; a, the unpaired 

 nostril on the middle of the head. See pp. 57, 60. 



