87 



Exoccipital (Figs.49A-B, 50). In cyprinids, the exoccipital is a paired endochondral 

 bone placed at the posterior portion of the neurocranium. The exoccipital forms the ventral 

 portion of the braincase's posterior wall and a part of the foramen magnum. The exocci- 

 pital sutures with the epioccipital and supraoccipital dorsally and basioccipital ventrally. 

 On the ventral view, it sutures with the prootic anteriorly, basioccipital medially, epiocci- 

 pital and pterotic anteriorly. The right and left exoccipitals suture one another mesially. A 

 cavum sinus impar is present between the dorsal margin of the basioccipital and the middle 

 portion of the ventral margin of the exoccipital. 



Posteriorly, the exoccipital extends laterally and expands medially in Phoxinus. The me- 

 sial edge of the exoccipital is laterally concave, therefore the foramen magnum is formed 

 between the two exoccipitals laterally and supraoccipital dorsally. A lateral occipital fo- 

 ramen is also formed on the exoccipital at the base near the medial margin. Viewed dor- 

 sally (after the supraoccipital and epioccipital removed), the left and right exoccipitals 

 form a butterfly-shaped structure^ Each of the two exoccipitals extends laterally to form 

 the "wing" portion of the "butterfly" (Figs.49A-B, 50), and the middle part of the bone 

 forms the "body" (Figs.49A-B, 50). At the median of the exoccipital is a thin and flat 

 plate-like portion (Figs 49A-B, 50) which meets its counter part at the medial side of the 

 exoccipital. The plate-like portion of the basioccipital bears two dorsal crests at the dor- 

 sal aspect of the bone (see also the description of basioccipital below). These two dorsal 

 crests of the basioccipital form a canal whereas the plate-like structure of the exoccipital 

 roofs the canal-like structure (Fig. 50). 



Variation of the exoccipital among the Phoxinus species includes shape of the wing por- 

 tion and the posterior margin of the plate portion. In P. phoxinus, neogaeus, and issykku- 

 lensis, the anterior margin of the wing-like structure is deeply concave (TS 78[1]); the 

 margin is shallowly concave in other species of Phoxinus (TS 78[0]). The posterior mar- 

 gin of the plate-like structure is convex in P. neogaeus, issykkulensis, and phoxinus (TS 

 79[1]); it is shallowly concave in P. eos, and erythrogaster, and deeply concave in other 

 species of Phoxinus (TS 79[0]). 



Fig. 50: Diagram showing the relationships between basioccipital and exoccipitals (anterior view) of 

 Phoxinus. 



