556 
CAPTAIN E. W. SHANN ON 
III. Chondrostei. 
(Accipenser rubicundus.) 
1. Shoulder Girdle. 
(l) The Lateral Muscle. 
The composition of the lateral muscle of Accipenser closely resembles that of the 
Teleostei, consisting, namely, of four series of hollow cones, two above and two below 
the lateral line. In the pectoral region four areas can still be traced ; of these, the 
two dorsal are completely severed from the two ventral by the passage of the hori- 
zontal septum which runs from beneath the lateral line to the transverse processes 
of the vertebrae. 
The dorsal moiety takes a double origin from the skull. The dorsalmost portion 
arises from the occiput just below the first dorsal spine and is formed by the upper 
halves of the dorsal backwardly directed cones, while the ventralmost arises from the 
exoccipital region and is formed by the ventral forwardly directed cones. This 
differs from the condition figured by Knauer for A. sturio, in which there appears 
to be only one area of origin for the dorsal moiety of the lateral muscle. 
The ventral moiety is very clearly divided into a latero-ventral and a mesio- 
ventral portion, which condition also appears in Knauer’s figure of A. sturio. In 
the former the fibres run parallel to the long axis of the body. This is also true for 
the mid-ventral fibres of the mesio-ventral portion ; but as the sides are reached the 
fibres become increasingly inclined, so that they make an acute angle with those 
of the latero-ventral portion. The slope of the fibres is from below anteriorly to 
above posteriorly. 
(2) The Posterior Muscles. 
When the lateral muscle reaches the pectoral girdle it becomes inserted to a large 
extent into the various parts of that structure. The muscles which are attached in 
this manner act as retractors of the girdle, and will be described in relation to the 
areas of the lateral muscle from which they are derived. 
(a) Retractor latero-dorsalis pectoralis ( r.l.d.p .). — Some of the superficial fibres 
break away from the main mass of the latero-dorsal portion of the lateral muscle and 
become inserted into the posterior border of the cartilaginous suprascapula. 
(b) Retractor latero-ventralis pectoralis ( r.l.v.p .). — The entire latero-ventral 
portion on reaching the girdle becomes inserted into the posterior face of the 
coraco-scapular cartilage. Not a fibre reaches the cleithrum. The muscle is seen 
in fig. 15. 
(c) Retractor mesio-ventralis pectoralis ( r.m.v.p .). — In the Sturgeon this muscle 
is divided into two portions ; the third or “ medius” portion figured and described in 
Elasmobranchii and Teleostei is not apparent. 
