48 
MM. supinator radii brevis (figs. 16 and 17, s.7.b.). (This muscle 
is systematised by Dr. Brooks* as a delaminated portion of the 
radial sector of the superficial extensor stratum.) 
It takes origin from the lowest part of the ectocondyle by a 
narrow origin, and is inserted into the outer surface of the shaft 
of the radius as far distally as the insertion of the M. pronator 
radii teres, encroaching upon both dorsal and palmar aspects of 
the bone. The posterior interosseous nerve (N. radialis prof.) 
appears at its dorsal border between it and the extensor ossis 
metacarpi pollicis. 
The muscle is relatively well developed in Votoryctes, occupying 
three-fifths of the shaft of the radius, and it is deeply placed 
under cover of the superficial extensor muscles. 
Cunningham notes the muscle as feebly developed in the three Mar- 
supial forms dissected by him,+ but Macalister notes it as well devel- 
oped in Koalat, occupying two-fifths of the shaft of the radius, in 
Wombat as occupying two-thirds of the bone, in the Tasmanian devil, 
Wallaby and Giant Kangaroo one-third, and in the Opossum only the 
upper fourth.§ 
In Dasyurus it is limited to the upper fourth,|| and MacCormick 
here notes an origin from the orbicular ligament of the radius. In no 
Marsupial has an ulnar region been described. 
In Hchidnaf the muscle is purely condylar in origin, and has an 
extensive insertion into four-fifths of the radius. 
In Ornithorhynchus it also possesses a condylar origin only, but its 
insertion is much less extensive, viz., into the upper third of* the 
external border, and slightly into the anterior surface of the radius 
(Brooks**). 
In Chlamydophorus the supinator brevis is poorly developed, and it 
is absent in Tatusia, but in several other Edentata it is large, e.g., 
Cyclothurus and Pholidotus.+t 
It is absent in the Talpidet? amongst the Insectivora, but present in 
the Hrinaceide.$$ It is present also in Rodentia|||| and Carnivora. 
The deep stratum of the extensor muscle of the antibrachium 
is represented in JVotoryctes by two muscles. The first is 
M. extensor ossis metacarpi pollicis (figs. 9-12 and 17 e.m.p.). This | 
isa broad muscle arising from both bones of the forearm between 
the supinator brevis on the radial, and the extensor indicis on 
the ulnar side, and extending as far proximally as the lower 
border of the anconeus externus, or even beneath it. At the 
wrist the muscle narrows and gives place to a fine tendon, which 
escapes obliquely from under cover of the common extensor of 
the digits, and, crossing the tendon of the extensor carpi radialis, 
it winds round the summit of the radial styloid and passes 
*i., page 1. tiv., page 16. {xxvili, page 130. § xxix., page 165, 
|| xxxvi., page 128. {| xxxix., page 387. **1., page 9. {7} xxvil., page 
255. tf xxvi., page 8ll. §§ ‘viii. page 395. |||| xl., page 400. 
