38 



ing the numbers of the hones : the yellow tint indicates the occipital segment or vertebra ; the 

 green the parietal segment ; the blue the frontal segment ; and the red the nasal segment. 

 The sense-capsules are denoted by crimson tickets ; the bones of the splanchno-skeleton by 

 brown tickets ; and those of the dermo-skeleton by white ones. 



The bones of the occipital segment form an upper, or ' neural,' and a lower, or ' haemal ' 

 arch : those of the neural arch encompass the medulla oblongata and cerebellum in the recent 

 fish, and are six in number. The first and lowest, called basioccipital (i), is a short, strong, 

 subrhomboidal bone, subcylindrical and truncate posteriorly, where it is excavated to form 

 the articular cavity, united with the corresponding hollow cone on the fore-part of the body of 

 the atlas ; the anterior pointed end of the basioccipital was wedged into the basisphenoid, 

 fitting and filling up the deep posterior cleft of that bone. The basioccipital offers on each 

 side a rough articular surface for sutural union with two lateral bones, the exoccipitals (2, 2) ; 

 behind which it received the anteriorly projecting base of the neural arch of the atlas, which 

 was wedged into the posterior angle between the basi- and exoccipitals, and was firmly united 

 to them by broad sutural surfaces. The exoccipitals present the form of oblong, subqua- 

 drate bones, thick, and with two rough, deeply-indented articular surfaces below, but ex- 

 panded and produced outwards above : they encircled the epencephalon, and completed the 

 contour of the foramen magnum. They are perforated for the passage of the nervi vagi, and 

 were articulated below with the basioccipital, behind with the neurapophyses of the atlas, 

 above with the superoccipital (3) and the paroccipitals (4), and in front with the petro- 

 sals (ta). 



The superoccipital (3) presents an elongated rhomhoidal form, and was articulated by an 

 inferior cellulo-sutural surface with the summits of the exoccipitals and the mesial angles of 

 the paroccipitals, forming the key-stone of the neural arch : it sends upwards and back- 

 wards 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. It advanced forwards and joined the frontal, pushing aside, as it were, 

 the parietals. 



The paroccipitalt (4, 4) were wedged into the angles between the ex- and super-occipitals : 

 they are of a conical form, with the base towards the cranial cavity, and the apex turned out- 

 wards and backwards. Their whole outer surface is here traversed obliquely by a prominent 

 ridge, ending at the lower and hinder projecting angle of the bone. The inner surface of the 

 paroccipital, like that of the exoccipital, is excavated for the lodgement of part of the poste- 

 rior and external semicircular canal of the large internal organ of hearing in Fishes. The 

 outer projecting process supported the upper fork of the first piece of the haemal or ' sca- 

 pular' arch. 



The second ring of bones, or that which encircled the mesencephalon (lobe of the third ven- 

 tricle and optic lobes), includes the basisphenoid (5), the alisphenoids (e), the parietals (r), 

 and the mastoids (). The basisphenoid () is connate with the presphenoid (9), forming 

 with it a long subtriedral bone (basi-presphenoid), usually bifurcate posteriorly, and more or 

 less expanded beneath the cranial cavity : it is then continued forwards along the base of the 

 interorbital space to near the fore-part of the roof of the mouth : its posterior extremity was 

 firmly wedged by a kind of double gomphosis into the basioccipital : its expanded part sup- 



