(1647). As the segments approach the tail in the air-breathing vertebrates, they are usually progressively simplified, first by the diminution, coalescence, and final loss of the pleurapophysis, next by the similar diminution and final removal of the haemal and neural arches, and sometimes also by the coalescence of the remaining central elements, either into a long osseous style, as in the Progs (fig. 333), or into a shorter flattened disk, as in many Birds. In Pishes, however, the seat of the terminal degradation of the vertebral column is first and chiefly in the central elements, which in the Homocercals, i. e. in those genera which, like the Perch (fig. 311), have a symmetrical bilobed tail, are commonly blended together, and shortened by absorption, whilst both neural and haemal arches remain with increased vertical extent, and indicate the number of the metamorphosed or obliterated centrums.

(1648). The anterior vertebrae of the spinal series are modified in their form and dimensions in proportion to the increased development of the anterior part of the cerebro-spinal axis, and that to such an extent, more especially in the mammiferous races, that their real nature and character are completely masked from ordinary observation, nevertheless guided by the principles above laid down, that the bones of the cranial portion of the spinal column conform in their essential arrangement with what has been observed in the rest of the vertebral series, and that the skull is in reality made up of the same elemental parts, modified, it is true, to a very remarkable extent, yet still recognizable, in accordance with just principles of philosophical induction, as the homo-logues of those described above.

(1649). The cranial bones, when examined by any unprejudiced observer, readily resolve themselves into four distinct vertebrae, which may be named, reckoning them from behind forwards, the - Occipital, or Epencephalic; Parietal, or Mesencephalic; Frontal, or Prosencephala Nasal, or Rhinencephalic.

* "On the Archetype and Homologies of the Vertebrate Skeleton." London: John Van Voorst.

(1650). The Occipital Vertebra, in the higher vertebrates, is represented by the occipital bone, in which all the vertebral elements are consolidated into one piece; in the Reptilia, however, it is by no means difficult to identify the several parts which enter into its composition. They are as follows: -

Centrum...

Basioccipital ....

5*

Neurapophyses . . .

Exoccipital ....

9

Spine.........

Supraoccipital . . .

8

Pleurapophyses . . .

Paroccipital ....

10

The composition of the Parietal Vertebra is -

Parietal centrum . .

Basisphenoid. .

6

Neurapophyses . . .

Alisphenoid.

Spine.......

Parietal.......

7

Pleurapophyses . . .

Mastoid....

12

The Frontal Vertebra consists of -

Frontal centrum . .

Prosphenoid and Entosphenoid,

Neurapophyses . . .

Orbitosphenoid . . .

14

Spine......

Frontol......

1

Pleurapophyses . . .

Postfrontal .......

4

The Nasal Vertebra is composed of -

Nasal centrum . . .

Vomer.....

16

Neurapophyses . . .

Prefrontal ....

2

Spine.........

Nasal..........

20

(1651). Thus far we are enabled to identify the cranial bones as being modified representatives of the spinal column, by allowing for that increased development of the neural elements here rendered necessary by the inordinate magnitude of the ganglionic centres of the cerebrospinal axis; but when we come to turn our attention to the constitution of the other portions of the chain, representing the lateral and inferior arches, or, in other words, the parapophysial and the hsemapophysial elements, together with the diverging appendages derived therefrom, the task becomes much more difficult. The labours of Professor Owen upon this interesting subject, unparalleled for depth of research, and exhibiting a grasp of philosophical argument rarely to be met with, have, however, satisfactorily revealed their real nature, and established beyond a doubt the alliances which exist between the elaborate structures in question, and the arches which exist under simpler conditions appended to the vertebral segments of the trunk.

* These numbers correspond with those that indicate the individual bones of the cranium in subsequent figures.

Elements composing the typical vertebrate skeleton (after Owen.)

Fig. 308. Elements composing the typical vertebrate skeleton (after Owen.) - The different elements of the primary segments are distinguished by peculiar markings: the neurapophyses by diagonal lines (n); the diapophyses by vertical lines (d); the parapophyses by horizontal lines (p); the centrum by decussating horizontal and vertical lines (c); the pleurapophyses by diagonal lines (pl); the haemapophyses by dots (A); the diverging appendages by interrupted lines (a); the neural spines and haemal spines are left blank.

The elements entering into the composition of the " archetype" are as follows: - 1, basioccipital, or basilar; 2, exoccipital; 3, supraoccipital; 4, paroccipital - these constitute the occipital, or encephalic vertebra of the cranium; 5, basisphenoid; 6, alisphenoid; 7, parietal; 8, mastoid - completing the mesencephalic, or parietal vertebra; 9, presphenoid; 10, orbitosphenoid; 11, frontal; 12, postfrontal - forming the prosencephalic, or frontal vertebra; 13, vomer; 14, prefrontal; 15, nasal - composing the rhinencephalic, or nasal vertebra; 16, the petrosal, or acoustic sense-capsule, interposed between the occipital and parietal vertebrae; 17, the sclerotic, or ophthalmic sense-capsule, lodged in a cavity or orbit between the frontal and nasal vertebrae; 18, 19, ethmo-turbinal, or olfactory sense-capsule, situated immediately in advance of its proper segment, which becomes variously modified to enclose and protect it; 20, palatine; 21, maxillary; 22, premaxillary; 23, 24,25, pterygoid; 26, malar; 27, squamous - constituting the haemal arch of the nasal vertebra with its diverging appendages; 28, tympanic; 29-32, mandibular, or lower jaw; 34-37, operculars - composing together the haemal arch of the frontal vertebra with its diverging appendages; the mouth opens in the interspace between the haemal arches of the first and second segments, its place being indicated by an arrow; 38, stylohyal; 39, epihyal; 40, ceratohyal; 41, basihyal; 42, glossohyal; 43, urohyal; 44, branchio-stegal - constituting the haemal arch of the parietal vertebra with its diverging appendages; 50, suprascapula; 51, scapula; 52, coracoid; 52', episternum; 53,anterior limb - completing the haemal arch of the occipital vertebra with its diverging appendages; 58, clavicle, the haemapophysis of the post-occipital vertebra; 62, ih'um; 63, ischium; 65-69, posterior extremity - constituting the haemal arch of the sacral vertebra (S), with its diverging appendages; 64, pubis, the haemapophysis of one of the lumbar (?) vertebrae.