1885. ] | Progressive and Retrogressive. se 
plectic, the inferior quadrate, and the articular. In the Batrachia, 
reptiles and birds, we have the quadrate and articular only, 
while in the Mammalia, ‘these elements also are wanting. 
The examination of the pectoral and pelvic arches reveels a 
successive modification of the adaptation of the parts to the 
mechanical needs of the limbs. In this regard the air-breathing 
types display wide diversity from the gill-bearing types or fishes. 
In the latter, the lateral elements unite below without the inter- 
vention of a median element or sternum, while in the former the” ~ 
sternum or parts of it, are generally present. Either arrangement 
is susceptible of much mechanical strength, as witness the Siiurgi 
fishes on the one hand, and the mole on the other. The güner i ee 
ous segments of the fishes’ pectoral arch must, howevef, be gan 
element of weakness, so that from a mechanical | 1 
must take the lowest place. The presence of s 
with both clavicle, procoracoid, and coracoi 
gives theaéXeptilia the highest place for, mec 
loss of the coracoid seen in the tailed Batrachjsd, sad loss of cora- 
coid and procoracoid in the Mayi ia, cone titute an element of 
weakness, The line is notti ‘uniform as Scent in this 
respect. E 
The absence of pel remely rudimental condition in 
fishes, places them. he line in this respect. The 
forward exte Batrachia andin the Mam- 
ckward direction in Reptilia, 
derived by desc mi ctly intermediate position in the 
Batrachia and 
rection must b 
over the posterio 
ed as having the mechanical advantage 
| tion, since it shortens the vertebral column 
and brings the po terior nearer to the anterior feet. The prev- 
alence of the latter condition in the Mammalia enables them to 
stand clear of t} » ground, while the Reptilia move with the abdo- 
men resting upon it. As regards the inferior arches of the pelvis, 
the Mammalia have the advantage again, in the strong bony me- 
dian symphysis « connecting the ischium and pubis.! This character, 
universal among the land Vertebrata of the Permian epoch, has 
been lost by the modern Bxtege@es Reptilia, and birds, and is re- 
tained only by the Mammalia. So the line, excepting the Mam- 
1 i i'an advantage as a protection during gestation. 
