446 Zoological Society. 



Owen, and duly considered by him prior to the publication of his 

 work ' On the Archetype and Homologies of the Vertebrate Ske- 

 leton,' 8vo, Van Voorst, 1848, from which he quoted the following 

 passages regarding their true nature and homologies. Viewing them 

 as processes from the cortical part of the centrum, Professor Owen 

 states : " The centrum may develope not only parapophyses, but in- 

 ferior median exogenous processes, either single, like those of the 

 cervical vertebras of saurians and ophidians (which in Deirodon scaber 

 perforate the oesophagus, are capped by dentine, and serve as teeth*), 

 or double (atlas of Sudis gigasf and the lower cervical vertebrae 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." 

 (p. 96.) 



To this view Professor Owen had been led chiefly by the coexist- 

 ence of these inferior exogenous processes in the anterior abdominal 

 vertebras of certain fishes with the true haemal arches, the nature and 

 modifications of which were so plainly demonstrated in the caudal 

 region of fishes. Besides the species cited in which these ' proces- 

 sus inferiores ' had been noticed by previous authors (Agassiz e. g. in 

 the case of Sudis gigas), Professor Owen had discovered other modi- 

 fications of the same nature, and referred to his description and figures 

 of the confluent subvertebral processes in the anterior trunk-vertebras 

 of the Bagrus tachypomus, a large siluroid fish (Vertebrate Archetype, 

 p. 92, pi. 1. fig. 3 ; Annals of Natural History, vol. xx. 1847, p. 217, 

 fig. 1). 



He had shown in his memoir on the so-called wedge-bones of the 

 Enaliosauria, that the subvertebral processes in fishes were homolo- 

 gous with those autogenous wedge-bones, with the exogenous infe- 

 rior processes of the cervical and dorsal vertebras of ophidians and 

 saurians, and with the body of the atlas in anthropotomy ; and in his 

 work on the Archetype, Professor Owen had summed up his views 

 of their nature in the following words : " The continuous bony plate 

 supporting those centrums was perforated lengthwise by the aorta, 

 offering another mode of formation of a haemal canal (c h), viz. by 

 exogenous ossification in and from the lower part of the outer layer 

 of the capsule of the notochord. The carotid haemal canal in the 

 necks of birds seems to be similarly formed ; and the neck of the 

 ichthyosaurus derives additional strength and fixation from appa- 

 rently detached developments of bone in the lower part of the cap- 

 sule of the notochord, at the inferior interspace between the occiput 

 and atlas, and at those of two or three succeeding cervical verte- 

 bras}. 



" The so-called ' body of the atlas ' in recent saurians, birds, mam- 

 mals and man, is the homologue of the first of these subvertebral 



* Jourdan, cited in Cuvier's Le9ons d'Anat. Compared, ed. 1835, p. 340, and 

 Odontography, p. 179. 

 t Agassiz in Spix, Pisces Brasilienses, 4to, 1829, p. 6. tab. B. fig. 8. 

 X Sir Philip de M. Grey Egerton in Geol, Trans. 2nd Ser. vol. v. p/187, pi. 14. 



