ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 679 



patch of soft pseudo-cartilage in the sections, and also the extreme posterior tip itself 

 is composed of soft pseudo-cartilage. This is of interest in view of Ayers and 

 Jackson's statement, which I venture to doubt, that this bar consists entirely of 

 pseudo-cartilage in Bdellostoma. The superior bar does, as Schaffer and myself 

 have pointed out. 



As confirmed by Stockard, the branchial skeleton of Bdellostoma is entirely extra- 

 branchial in all cases except that of the cesophago-cutaneous duct (and the one 

 described below), where, as in Myxine (7, fig. 11), the cartilage extends to, and 

 slightly spreads out on, the oesophagus. Stockard, however, believes that the 

 Myxinoids " are contrasted with fishes as having extra branchial skeletons on rather 

 flimsy grounds" (30, p. 509), pointing out that the extra-branchial position may well 

 be secondary owing to the development of pouched gills, the pouches appearing before 

 the cartilages and preventing them spreading on to the gut as in the case of the 

 ductus cesophago-cutaneus. Stockard describes the disappearance of three pairs of 

 gill clefts behind the mouth — a hyomandibular pair and two pairs of post- 

 hyomandibular — a fact which must be reckoned with in working out the morphology 

 of the Myxinoid skeleton. In this connection I may point out that Schaffer 

 describes a rudiment of soft cartilage on the right side of the oesophagus immediately 

 behind the gills (28, p. 236, fig. 25). I find this also in my large series of sections. 

 It extends from section 3095 to 3110, and commences at about opposite the posterior 

 margin of the external opening of the ductus oesophago-cutaneus, and therefore 

 immediately behind the gill region. It is embedded in the submucosa of the 

 oesophagus (of course, under the constrictor muscle) at the middle of its lateral 

 surface. The interesting feature of this small piece of cartilage is that it undoubtedly 

 corresponds to the oesophageal rod on the left side which I have called x 1 (Part I., 

 figs. 11 and 13). It is exactly opposite this rod in the sections, and in the same layer 

 of the oesophagus, but extends eight sections behind it. There is thus a visceral 

 portion of the branchial skeleton on the right side as well as on the left, which 

 coincides with Stockard's views mentioned above. 



September 10, 1908. 



LITERATURE. 



[In this list only those works are included which contain original observations on the skeleton of 

 Myxinoids — exclusive of the teeth.] 



(1) Allis, Anat. Anz., Bd. xxiii. p. 259, 1903. General observations on skeleton of Bdellostoma. 



(2) Ayers and Jackson, Jour. Morph., vol. xvii. p. 185, 1901. Reprinted : Bull. Cincinnati Univ., Ser. ii., 



vol. i., No. 1, 1900. General description of skeleton of Bdellostoma. 



(3) Bull. Cincinnati Univ., Ser. ii., vol. i., No. 1, 1900. Morphology of skeleton of Myxinoids. 



(4) Bubne, Proc. Zool. Soc, 1892, p. 706. Branchial basket in Myxine. 



