ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 767 
dorso-median fold of its lining. This lies near the external nasal opening, and is 
supported by a delicate strip of free cartilage (cp. fig. 1). It is probably not the 
structure referred to by Parxer as the nasal valve, which is merely an adventitious 
fold of the lining mucous membrane of no importance, and occurring capriciously at all 
parts of the tube. It is, doubtless, a very small tentacle. Latero-ventrally, on each 
side, the first four rings were all connected up by a longitudinal bar (fig. 1).* In some 
serial sections prepared by the late Dr Poxtarp and lent me by the late Prof. 
G. B. Howes, the third arch sent back and the fourth arch despatched forwards processes 
which overlapped but did not fuse. Further, the fifth and the sixth arches sent 
forwards prominent projections which did not, however, quite reach the arches in front. 
As also described by J. MULLER and Ayers and Jackson for Bdellostoma, the lateral 
connecting bar in front sends forwards a strip of cartilage (fig. 1), which the latter 
authors found in one case to fuse with its fellow of the opposite side in the mid-ventral 
line. These cartilages therefore express a tendency to form a complete ring round the 
external nasal opening, but may represent a pair of vestigial tentacles. Arches five to 
eight are quite independent; but the last three are connected ventro-laterally by a 
longitudinal bar with each other, and the last with the nasal capsule (fig. 1). Dorsally 
the tenth arch is Y-shaped, and also connected in the middle line with the eleventh 
(fig. 5). The last or eleventh arch is fused with the anterior transverse bar (a. t. b.) 
of the olfactory capsule at five places—by one median and two lateral pairs of rods 
(figs. 1, 5, and 6). Asa result, four fenestrae are formed—a small dorsal and a large 
lateral one on each side. This corresponds exactly with the condition in Bdellostoma 
described by J. MULuER,{ and also practically with the figures of Ayers and Jackson. 
In a series of sections of a 25 cm. Hag the following variations from the above 
description may be noted (cp. fig. 6). Arches 7 and 8 were fused ventrally, but 9, on 
the other hand, was independent, although underlapping 10 below. Ring 10 joined 
not directly with 11 to complete the large anterior fenestra (but it ded on the other 
side), but with the process connecting the latter ring with the anterior transverse bar 
of the nasal capsule (a. ¢. b.). At this region an additional independent cartilage was 
_ present on the left side only, and there was also an additional fenestra dorsally 
separating rings 10 and 11. Anteriorly, this specimen agreed even in the smaller 
details with the above description, and hence these posterior variations become the more 
interesting. é 
The olfactory capsule itself (n. c.), like the nasal rings, protects only the roof and 
sides of its cavity. In front the latter is connected with the lumen of the nasal tube, 
and below with the hypophysial canal. It is shut off posteriorly from the cavity of the 
brain-case by the double wall of the cranium, as described by J. Mtiier. The capsule 
consists essentially of a series of longitudinal rods fused in front and behind respectively 
with an anterior (a. t. b.) and a posterior (p. t. b.) transverse bar. There are nine of these 
* Tn one series of sections the fourth ring was not included on one side. 
+ Op. cit., p. 109. 
