ANATOMY OF VERTEBRATES. 107 



nection and displaced backward. The humeral segment of the 

 fore limb is rarely developed in fishes ; the radius, 54, and ulna, 55, 

 are directly articulated with the coracoid, and are commonly much 

 more broad than long. 



Some of the special characters and modifications of the bones 

 of the head will next be briefly noticed. 



The articular cup for the atlas varies from the deep conical 

 excavation seen, fig. 77, 1, in the Cod, to the almost flat surface 

 in the Halibut ; it is rare to find, as in the Pipe-fish {Fistularia), 

 the basioccipital presenting a convex surface for articulation with 

 the body of the atlas ; or to find this centrum confluent with the 

 basioccipital, as in Polypterus. In many fishes the under part 

 of the basioccipital is expanded and excavated ; in the Carp, 

 the under part is produced into a broad triangular plate, fig. 83, 1, 

 which supports the large upper pharyngeal grinding tooth ; in 

 the ganoid Lepidosteus, the basioccipital developes two plates 

 from its upper and outer angles, which complete the foramen 

 magnum and support the exoccipitals above. The exoccipitals, 

 fig. 77, 2, are perforated for the passage of the nervi vagi, some- 

 times for the first spinal or hypoglossal nerve ; the foramina being 

 unusually large in the Carp tribe, fig. 83, 2, where they relate 

 also to the connection of the air-bladder with the organ of hearing, 

 by means of the ossicles, a, b, c, d, and e. 



In some fishes, e.g. Perca, fig. 84, 2, the exoccipitals send 

 backward articular processes modified to allow a slight move- 

 ment upon the anterior articular processes of the atlas. Like 

 the neurapophyses of the trunk in some fishes (e.g. Lepidosiren, 

 Thynnus, Xiphias), the bases of the exoccipitals expand, and 

 meet upon the upper surface of the basioccipital, and immediately 

 support the medulla oblongata. 



The superoccipital, fig. 77, 3, iisually sends upward and back- 

 ward a strong compressed spine from the whole extent of the 

 middle line, and a transverse ' superoccipital ' ridge outwards 

 from each side of the base of the spine, to the external angles of 

 the bone. In most fishes this bone advances forward and joins 

 the frontal, pushing aside, as it were, the parietals, as in fig. 76, 

 3 ; in Balistes the produced part of the superoccipital is even 

 wedged into the hinder half of the frontal suture. In the Carp, 

 on the contrary, the anterior angle of the superoccipital is trun- 

 cated, forming the base of the triangle, and is articulated by a 

 lamboidal suture to the parietal bones, fig. 83, 7, which here meet 

 at the mid-line of the skull, and the upper part of the occipital 

 spine is low and flattened. The superoccipital is also separated 



