PARIETAL SEGMENT, OR VERTEBRA. 43 



have become "confluent," and were not "connate." The 

 centrums of the two middle segments of the fish's skull 

 are connate, and the little violence above recommended is 

 requisite to detach the penultimate segment of the skull. 

 AVhen detached, the bones of it are seen to be so arranged 

 as to form a neural and a haemal arch. In the neural arch 

 the centrum, neurapophyses, diapophyses, and neural spine 

 are distinct : moreover, the neural spine in the cod, and 

 many other fishes, is bifid, or split at the median line.^ 

 The centrum is called " basisphenoid," No. 5 ; the neura- 

 pophysis, " alisphenoid," No. 6; the neural spine, "pa- 

 rietal," No. 7; and the diapophysis, "mastoid," No. 8. 

 The alisphenoids protect the sides of the optic lobes, and 

 the rest of the penultimate segment of the brain; the 

 mastoids project outwards and backwards as strong trans- 

 verse processes, and give attachment to the piers of the 

 great inverted haemal arch. Before noticing the structure 

 of this, I may remark that, in the recent cod-fish, the case, 

 partly gristly, partly bony, which contains the organ of 

 hearing, is wedged in between the last and penultimate 

 neural arches of the skull. The extent to which the ear- 

 case is ossified varies in different fishes, but the bone is 

 always developed in the outer wall of the case. In the 

 cod-fish it is unusually large, and is called "petrosal," 

 No. 16 ; it forms no part of the segmented neuroskeleton. 

 In the organ which it contributes to inclose, there is a 

 body as hard as shell, like half a split almond ; it is the 

 "otosteal," No. 16, or proper ear-bone. 



The hcemal arch consists of a pleurapophysis and a 

 haemapophysis on each side, and a hernial spine; but all 

 these elements are subdivided ; the pleurapophysis into 

 two parts, the upper one called "epitympanic," 28, a 



' " Archetype Vert. Skel.," p. 11, Fig. 2. 



