264 REPORT—1846. 
Summary of modifications of corporal vertebre.—To sum up the kind and 
degree of modification to which the several elements of the primary segnients 
of the endoskeleton of the trunk are subject, without masking their general 
homology, we may commence with the centrum; and first, as to its existence. 
It is wanting, as an ossified part, in the atlas of the wombat and koala*, in 
which it remains permanently cartilaginous: in the petaurists, kangaroos, 
and potoroos, ossification extends from the bases of the neurapophyses into | 
this cartilage, but the neural arch or ring long remains interrupted by a me- 
dian fissure below. In man the rudimental body of the atlas is sometimes 
ossified from two or even three distinct centrest. The centrums at the oppo- 
site extremity of the vertebral column in homocercal fishes are rendered by 
centripetal shortening and bony confluence fewer in number than the per- 
sistent neural and hemal arches of that part. The centrums do not pass 
beyond the primitive stage of the notochord in the existing lepidosiren, and 
retained the like rudimental state in every fish whose remains have been found 
in strata earlier than the permian era in Geology, though the number of 
vertebrz is frequently indicated in Devonian and Silurian ichthyolites by the 
fossilized neur- and hem-apophyses and their spines}. The individuality of 
the centrums is sometimes lost by their mutual coalescence without short- 
ening. 
‘Although the normal form of the centrum is cylindrical, it may be cubical, 
conical, hour-glass shaped, like a longitudinal bar, like a transverse bar, like 
a depressed or a compressed plate, like a ploughshare, &c. The co-adapted 
terminal surfaces of the centrum may be flat, slightly concave, deeply con- 
cave, cupped or conical, concave vertically and convex transversely at one 
end and the reverse at the other end§; or the fore-end may be concave and 
the hind-end convex||; or the reverse] ; or both ends may be convex**; 
or both ends produced into long pointed processes with intervening deep fis- 
sures, so as to interlock together by a deeply dentated sutural surfacet+-. 
The centrum may be quite detached from its neural arch (atlas of siluroid 
and many fishes), and from its hemal arch (atlas of most fishes). 
The centrum may develope not only parapophyses but inferior median 
exogenous processes, either single, like those of the cervical vertebre of 
saurians and ophidians (which in Deirodon scaber perforate the cesophagus, 
are capped by dentine, and serve as teeth {{); or double (atlas of Sudis gigas §§ 
and the lower cervical vertebra of many birds) ; or the fibrous sheath of the 
notochord may develope a continuous plate of bone beneath two or more nuclei 
of centrums, formed by independent ossification in the body of the notochord ; 
these nuclei being partially coherent to the peripheral or cortical plate. The 
vertebral centrum often shows the principle of vegetative repetition by its 
partial ossification in the form of two or three bony rings, which answer to a 
single neural arch (Heptanchus|\\|), or by three osseous discs, one for each 
* Art. Marsupialia, Cyclopedia of Anatomy and Physiology, vol. iii. p. 277, fig. 99. 
+ Meckel, Archiv fiir Physiol. i. taf. vi. fig. 1. 
t See the admirable Monograph by Agassiz, Sur les Poissons Fossiles du Systeme Dé- 
vonien, 4to, 1846. § Most birds. 
|| Existing saurians and ophidians. 
g Extinct saurian called ‘ Streptospondylus ;’ existing Salamandra, Lepidosteus. 
** 4th cervical of Emys, Bojanus, Anat. Test. Europ., tab. xiv. fig. 51,4. Ist caudal of 
crocodile. 
+t Cervicals or anterior trunk-vertebre of Fistularia. 
+t Jourdan, cited in Cuvier’s Lecons d’Anat. Comparée, ed. 1835, p. 340, and ‘ Odonto- 
graphy,’ p. 179. 
§§ Agassiz in Spix, Pisces Brasilienses, 4to, 1829, p. 6, tab. B, fig. 8. 
\||| Miller and Agassiz, in Recherches sur les Poissons Fossiles, t. iii. tab. 40°, fig. 1. 
