?S. Opercle. 66. Centrum.
26. Subopercle. 67. Neuropophyses.
27. In ie. 68. Neural spine.
18.
69. Hiemapopbyses.
M. Angurar. 70. Banal spine.
30. Dentary. 71. Zygapophyses.
31. Ma. 72. Parapophyses.
12. Premalary. 73. Rib.
33. Interhyal. 74. Epipleural spines or 34. Epihyal. Epipleurals.
35. Cerathoyal. 75. Intemeural spines or 36. Basihyal. Interneurals.
37. Glossohyal. 76. Dorsal rays and spines.
38. Urohyal. 77. InteThies:al spines or 39. Branchiostegal. Interhiemals.
40. Basibranchials. 78. Anal rays and spines.
41. Hypobninchials. 79. HYPural.
42. Ceratobranchials. 80. Caudal rays.
43. Epibranchials.
The skull of the fish consists of a great va riety of bones, the names of which are given on the accompanying plate. In the naming of the bones of the head and shoulder-g:rdle of fishes great confusion has arisen. This is due mainly to ineffective attempts to trace the homologies of these bones with bones occupy ing similar positions in the skeleton of man; while, no doubt, the skeleton of the higher vertebrates is derived by descent from that of primitive fishes, homologies can be traced only in a general way. The skull of the one corresponds to the skull of the other, but the specialized fishes have developed many more bones than were found in ancestral types and many more than exist in man. In like fashion the shoulder girdle in the one represents that of the other, but the fish has more bones than can be cov ered by the names scapula, coracoid and clavicle.
In the accompanying plate of the bones of the striped bass (after Edwin Chapin Starks) we use the terminology now adopted by Gill, Starks and most American osteologists. The bones of the head may be roughly divided those of the cranium, the jaws, the suspens4r, (of the lower jaw), the gill-structures and membrane-bones. The latter, on the of the head, are formed by ossification of tik skin and have no homologies among the highs vertebrates. They are also wanting in sharks and lampreys.
The shoulder-girdle in the lower forms cn!- sists of an arch of cartilage. In the of specialization this is divided into sevr, pieces as indicated in the plate. It is separl from the skulls in sharks and skates, as in higher vertebrates. In the typical fishes
uppermost bone (post-temporal) is joined the temporal bone of the cranium. In sa groups it is immovably consolidated with latter. In the eels, by a process of degem tion, it loses its connection with the skull. Ta pelvis in fishes usually consists of a single bone on either side, more or less filly coalesced with its fellow.
The vertebral column may be divided into abdominal and caudal vertebra. The have their lower processes divergent, to In clude the body cavity. In the caudal the haemal processes unite, leaving opening onh for the hmmal canal or place of the large artery. The neural processes of each vertebra unite above the centrum or body of the verte bra, leaving space for the spinal cord. The pointed upper continuation of each is called the neural spine as the downward projection is called the hamal spine. In the primitive fishes generally the tail is extended to a point, the last vertebra being progressively reduced and the caudal fin lying below the axis. This forms the heterocercal tail, characteristic of the shark, sturgeon and of various extinct types of fish. In the modern types, the vertebrae cease more or less abruptly at the base of the caudal fin, the tail being homocercal.
The number of vertebra is least in the more specialized, spiny-rayed fishes. In several families of these there are 24 vertebra, 10 ab dominal and 14 caudal. This number is much greater in all the soft-rayed and all the primi tive forms. It is also materially increased by a process of degeneration in most arctic or sub arctic derivatives from these families and in general in fresh-water, pelagic and deep-sea forms derived from forms having 24 verte bra. In cold waters, fresh waters and the oceanic abysses, the processes of competition called natural selection are apparently less active and we have the of reduc tion of parts with the increase in number of similar structures. This statement must be re garded as hypothetical. While it is a rule almost universal, that in each group cold water forms have the vertebra smaller and more numerous, we have yet to find an adequate ex planation of the cause of the phenomenon.