1894.] MTOL06T OF EODENTS. 281 



Owen describes the left rectus in Capromys fournieri as passing 

 through a slit in the right, the right being therefore both super- 

 ficial and deep to the left ; this is practically what I found in 

 Octodon, the only difference being that each rectus rose by two 

 heads, the two belonging to the left muscle passing together between 

 the two belonging to the right. In Myopotamus, according to 

 Martin, the four heads alternate, one of the left being most super- 

 ficial. The rectus in Aulacodus was not noticed. 



Mivart and Murie * describe a well-marked decussation in the 

 Agouti ; this I did not see ; indeed, in none of the animals examined 

 was there any clecussatioQ approaching in distinctness that found 

 in the Octodontidae. 



Psoas Parvus. — The psoas parvus varies very much in develop- 

 ment among the Hystrieomorpha ; it rises from the sides of the 

 bodies of a variable number of lumbar vertebrae, and is inserted 

 into the ilio-pectineal eminence on the brim of the pelvis. 



In the Dipodidse the muscle is large and rises from all the 

 lumbar vertebrae except the last one or two (B. agyptius, D. 

 hirtipes). 



Among the Octodontidae it is small and rises from the anterior 

 3 or 4 vertebrae, and from the crura of the diaphragm in Aulacodus, 

 Myopotamus, and Capromys. In Octodon it was absent. 



In the Hystricidse it has the same arrangement (Rystrix, 

 Sphingurns). 



In the Chinchillidae the muscle is large, and rises from all or 

 nearly all the lumbar vertebrae (ChincJdlla, Lagostomus). 



In the Dasyproctidae it is small, and rises from two or three of 

 the central lumbar vertebne. 



In the Caviidae it is very small and apparently often absent. In 

 one specimen of Ceredon it was absent, while in another it rose 

 from the 4th and 5th vertebrae. In two specimens of Cavia 

 cohaya it was present, in one it was absent. 



In the Sciuromorpha it is always present and well marked ; it 

 usually rises from the anterior four or five lumbar vertebrae. 



Psoas Magnus. — This muscle shows much less variation than 

 that of the psoas parvus. It rises from the sides of the bodies and 

 ventral surfaces of the transverse processes of all the lumbar 

 vertebrae, and occasionally from the first sacral. The muscle is 

 usually more or less distinctly divided into an inner and outer 

 part by some of the branches of the lumbar plexus, this division 

 being specially well seen in Spermophilus. It has the usual 

 insertion as in Man. 



Iliacus. — The iliacus rises from the iliac surface of the ilium ; it 

 soon joins the psoas, with which it is inserted. Nothing charac- 

 teristic was noticed about it in the different animals examined. 



Quadratus Lumborum. — This muscle rises from the sides of the 

 bodies of the posterior dorsal vertebra3, usually the last six, and 

 from the tips of the transverse processes of the lumbar vertebrae ; 

 it is inserted by a narrow tendon into the ventral surface of the 



1 P. Z. S. 1866, p. 383. 



