THE MYOLOGY OF AMIURUS CATUS. 349 
A greater number of segments (5) appear, however, to enter into its 
composition. It may be pointed out that the direction of the fin is 
not exactly similar to that of the pectoral fin, which is more normal 
in this regard. One may suppose, however, that the absence of a 
true pelvic arch has something to do with this. If one imagines a 
partially aborted pectoral arch in the normal position, with the me- 
tapterygials, etc., directed somewhat backwards, one would have an 
intermediate stage between what obtains in the pectoral and pelvic 
fins of the Teleosts. 
The Dorsal Fin Muscles.—The innervation of the erectores and 
depressores of the dorsal fin is similar to that for the supracarinales, 
t.e., the ram. lat. trigem. acts as a collector for the dorsal branches of 
the spinal nerves, and gives off branches to the muscles. It would 
seem, from the relations of these muscles, and also from their inner- 
vation, that they are serially homologous with the supracarinales. 
Dohrn’s views! on the subject of the impaired fins receives confirma- 
tion from the paired nature of the muscles, and still more from the 
fact that a blood-vessel passes horizontally along through the base of 
each ray, the ray splitting readily upwards from this channel, point- 
ing to a coalescence of two parts, one on either side of the middle 
line, in the formation of the fin. 
The Anal Fin Muscles.—With regard to the erectores and depres- 
sores of this fin, the remarks made on those of the fin just described 
apply equally well. They are really serially homologous with the 
anfracarinales. The lateral muscles of the anal fin are, however, of 
an entirely different nature. Their innervation is from a superficial 
plexus similar to that supplying the erectores and depressores. The 
muscles lie completely outside the fascia covering the lateral muscles 
of the trunk, and the plexus which supplies them is peculiar in being 
in a similar manner superficial and formed from a plexus. The pro- 
bability is that the muscles are dermal in their nature, and that the 
plexus is a secondary one, produced from the deeper plexus already 
present as the muscles gradually developed from the dermal tissue. 
The Caudal Fin Muscles.—These are nearly all modified portions 
of the lateral musculature of the trunk. The intrinsic muscles are 
not, however, but must probably be referred to the class of dermal 
muscles. The innervation of the dorsal portions of the fin and of the 
anterior continuation of that dorsal portion is interesting in showing 
the relations of these parts to the dorsal and adipose fins. 
1 Dohrn.—Loe. eit. 
