of the Fishery Board for Scotland. 



281 



ossification leaving a space between it and the margin of the frontal above. 

 The anterior part of this foramen is bounded by the lateral arm of the 

 frontal, and its upper straight border by the outer edge of the main body 

 of the frontal behind its process. Immediately within the foramen lies 

 the upper part of the hind brain, which, therefore, at this point is entirely 

 unprotected on both sides except for the thin skin which covers these 

 foramina. 



Anteriorly the cartilage is partly replaced by the ethmoidal ossifi- 

 cation (eth.). The anterior (prenasal) end of the cartilage is ossified 

 as a nodular mass, so that no exact distinction can be made between what 

 seems to represent both mesethmoidal and supraethmoidal elements. The 

 bone terminates anteriorly in a median rounded tip, which is cut off from 

 the remainder by a thin strip of cartilage, and represents the labials (lb.). 

 Behind these, and on the dorsal surface, there rises a strong bony median 

 ridge (eth.r.), which anteriorly ends abruptly, and passing backwards dies 

 away, dividing into two flat supraethmoidal splints (figs. 18, 20, 22, eth.) 

 which lie over each edge of the cartilaginous roof, and are overlapped at 

 their tips by the frontals. The bony mass below the anterior end of 

 its dorsal ridge sends downwards and outwards on each side a 

 pair of (laterally) thin processes formed on the posterior faces of the 

 cartilaginous laterally projecting prenasal cornua (figs. 18, 21, 22, cor.). 

 The cartilage under the central ridge is ossified to form the nodular 

 mesethmoidal mass described, and it is closely underlapped by the head of 

 the vomer. 



In close contact with the tips of the ethmoidal posterior processes is a 

 pair of antorbital (ectethmoid) ossifications (ec.eth.) standing out with a 

 sharp edge, and forming the anterior boundary of the orbit, just as the 

 sharp anterior part of the sphenotic does the posterior. They send a 

 short supraorbital process backwards, and in the shad or sometimes even 

 in the herring, the median posterior portions are practically conjoined, and 

 meet with the prolongation of the orbito-sphenoid. Just within the nostril, 

 and lying immediately under the skin over the nasal cavity, and therefore 

 unattached to any other bone, is a very small nasal bone, sometimes so 

 slightly developed in the herring as to be difficult of demonstration ; it is 

 more pronounced in the other species. 



Running along nearly the whole floor of the skull is the parasphenoid. 

 Its anterior end reaches to beneath the nasal cavities, and is underlapped 

 by the vomer. This vomer (yo.) is a narrow elongated somewhat dagger- 

 shaped bone, closely applied at its anterior end to the ethmoidal and labial 

 ossifications. Its anterior third bears on its ventral surface an elongated 

 oval-shaped ridge bearing a double irregular row of teeth, while another 

 and opposite but shorter ridge stands up from it dorsally, fitting into the 

 mesethmoid. 



For the anterior two-thirds of its length the parasphenoid (p.sph.) is 

 narrow and flattened from above downwards, and is developed on the thin 

 rod of cartilage previously described as extending back beneath the orbits. 

 Near the cranial mass the parasphenoid becomes thicker, and articulates 

 with an irregular suture by a pair of short upstanding processes (fig. 18, p.sph.) 

 with the prootics. From this point a pair of very thin rather deep laminae 

 (p.sph.w.) pass backwards. Their upper edges are applied to the ventral 

 ridges of the prootics and basioccipital, and they therefore further deepen, 

 and although scarcely or just touching in the middle line, partly close in 

 the eye muscle canal. These 'wings' extend as far back as the junction 

 of the second and third vertebras, terminating in somewhat rounded tips, 

 which form, therefore, the most posterior projection of the skull bones. 

 It is immediately behind these that the swim-bladder gives off its two 



