



II -..i. 



FiM 



674 



MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



zygapophysis of any preceding vertebra is thus received in the recess between 

 pre-zygapophysis and its overarching process. I r.tn uncertain of the lioinology 

 of this process tlius surmounting the true diapophysis ; but it may he sup- 

 posed to include both anapophysial and metapophysial elements. Owen says 

 (Anat. Vert, ii, 365) that in Rodentia " the met- and un-apophyses commence 

 by a common tubercle at the fore part of the dorsal series : the anapophysis 

 begins to be distinct at the back part of the series, and the metapophyses to 

 project from above the anterior zygapophysis ; botli processes are usually 

 well developed in the posterior dorsal and lumbar vertebrae ; the diapophysis 

 subsides in the posterior dorsals, and is lengthened in the lumbars by a coal- 

 esced riblet (pieurapoj)hysis) ". Whatever the theoretical homologies may 

 be in this case, the actual formation is as above given ; the structure changes 

 suddenly on the 11th dorsal, from which point backward on the 12th and 

 13th dorsals, and several succeeding lumbars, both anapophyses and meta- 

 pophyses may be recognized, separate from each other and distinct from the 

 probably di-pleur-apophyses of the lumbars, which, with the anapophyses, 

 constitute the so-called transverse processes of the lumbar series. On the 

 11th dorsal, the last vestige of a true dia|)ophysis has disappeared, and with 

 it has gone (he above-described plate of bone, which surmounted it in the 

 antecedent dorsals. The side of the bone becomes smooth and erect ; a long, 

 pointed anapophysis projects from the hinder border of the bone, lying 

 parallel with the axis of the spinal column. A small, though undoubted 

 metapophysis is given off from the pre-zygapophysis, and such metapophysis 

 and pre-zygapophysis are together locked in between the anapophysis and 

 post-zygapophysis of the antecedent vertebra. Coincidently with tliis modi- 

 fication, the planes of the articular faces of the zygapophyses change from 

 nearly horizontal to nearly vertical; rendering the sum of the differences 

 between tlio 10th and 11th dorsals greater than that subsisting between the 

 last dorsal acid the first lumbar, even taking into consideration the different 

 formation of the transverse processes of the lumbar series. Such formation 

 continues through the remainder of the dorsal series. 



In the lumbar series, we directly meet, on the 1st lumbar, with a differ- 

 ently constituted " transverse" process. The ribs having ended witii the last 

 dorsal, the lumbars develop at once a lateral plate-like " transverse " process, 

 which may be theoretically regarded as a di-pleur-apophysis. This plate 

 increases in size with successive lumbars, projects with each one more and 



