89] THE SKULL OF AM lURUS— KINDRED 89 



Within the cranium, the anterior margin of the bone interdigitates with the 

 prootic as far laterally as the base of the lateral septum semicircularis, and 

 medially as far as the roof of the recessus sacculi (Fig. 7). Behind the lateral 

 septimi semicircularis, the exoccipital lamella extends laterally over the floor 

 of the recess for the posterior semicircular canal and part way up its posterior 

 wall. It is separated by cartilage from the more lateral and dorsal pterotic 

 and epiotic lamellae. The inner ends of the foramina for the glossopharyngeal 

 and vagus nerves lie in this part of the bone. Medial to the posterior recess, 

 a part of the bone extends out dorsally and, with the supraoccipital, forms a 

 wall between the recess and the cavum cranii. At the base of this wall a hori- 

 zontal process extends medially above the recessus sacculi and meets a similar 

 process from the other side to form the roof of the sinus impar. The ventral 

 surface of this process meets a splint from the basioccipital, which forms the 

 side wall of the cavum sinus imparls and the roof of the recessus sacculi of that 

 side. The lateral wall of the recessus sacculi is formed by exoccipital and basi- 

 occipital lamellae. 



The foramen for the hypoglossus nerve lies posterior to and above the lateral 

 end of the horizontal process. The latter forms the ventral margin of the fora- 

 men magnum, and between it and the dorsal surface of the basioccipital the 

 sinus impar leaves the cranium to divide into the atria sinus imparls. The 

 posterior ventral end of the exoccipital is fused with the basioccipital. 



The exoccipital bone is one of the most constant bones throughout the ver- 

 tebrate series. It is fairly well developed in all higher groups of ganoids, 

 although considerable cartilage remains between the inner and outer lamellae. 

 As in Amiurus, it forms the floor and side walls of the foramen magnum and 

 there is usually a gap of cartilage in the dorsal margin of the foramen where in 

 Amiurus the supraoccipital lies. In some teleosts — Salmo, Citharinus, and 

 Catostomus — the cartilage persists in this region. 



The anterior margin of the bone usually encloses the ninth and tenth nerves, 

 either in a single foramen or an anterior and a posterior opening, and ossifica- 

 tion usually starts in the cartilage around these foramina. The intercalated 

 nerves are secondarily enclosed in the posterior part of the bone, as in Amiurus. 

 In the Cyprinidae (Sagemehl, 1891), this foramen is larger and is divided into 

 two parts, one for the ventral and the other for the dorsal root of the nerve. 

 The dorsal foramen is very large in Catostomus, Cyprinus, and Cobitis, and, 

 above the nerve, is filled with lymphatic tissue continuous wath the contents of 

 the saccus paravertebralis of the Weberian apparatus. Sagemehl claims that 

 the larger has arisen by the fenestration of a single original foramen such as 

 occurs in Amiurus. 



Ostariophysian teleosts have a median horizontal process on each exoccipi- 

 tal, which, with its fellow of the opposite side, forms the cranial floor above 

 the sinus impar and the recessus sacculorum. Actual contact of the exoccipi- 

 tal with the surrounding bones is not usually the case, because cartilage per- 



