64 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
three muscular sheets: the external and the internal oblique, and 
the transversalis, all having their origin ventral to the back muscle 
on the outer border of the ribs. I was unable to demonstrate their 
insertion with certainty, but it was apparently on the edge of the rec¬ 
tus abdominis by aponeurosis. It is probable that this aponeurosis 
envelops the rectus, conforming to the usual arrangement in allied 
forms. The fibers of the external oblique run from their origin 
postero-ventrally, those of the internal oblique run from their origin 
antero-ventrally, and those of the transversalis, which is interior to 
the internal oblique, run nearly dorso-ventrally, slanting a little to 
the anterior. All these muscles are divided into segments by myo- 
commata. 
The rectus abdominis (pubo-hyoideus) is a paired muscle similar 
to the back muscle. It arises from the postero-ventral border of 
the first cerato-branchial and inserts on the antero-ventral surface of 
the pubo-ischium in the mid-ventral region, separated from its fel¬ 
low by the linea alba, and covered in the region of the shoulder 
girdle by the coracoids and the pectoralis. 
In the caudal region the vertebrae are flattened and the dorsal 
muscle becomes compressed and extended ventrally so that it forms 
a thin lateral sheet, that meets its fellow in both the mid-dorsal and 
mid-ventral lines. Some of its fibers are attached to the lateral sur¬ 
face of each vertebra. 
9 
Muscles of the Fore Limb. 
Dorsal Surface. 
In describing the dorsal muscles of the fore limb, I shall divide 
them into two groups, the extrinsic (those that are attached by one 
end to the girdle or limb) and the intrinsic (those that are attached 
at both ends to the girdle or limb). 
Extrinsic muscles. — The levator scapulae is a short thick muscle, 
whose origin I was unable to ascertain, although it is probably the 
postero-dorsal border of the skull as in Proteus. It inserts on the 
antero-dorsal border of the inner surface of the scapula and serves, 
with the trapezius, to draw the scapula upward and forward. 
The trapezius (cucullaris), a thick fan-shaped sheet, arises with its 
fellow from a mid-dorsal aponeurosis, anterior to the scapula, and 
