36 PROF. T. H. HUXLEY ON CERATODXJS FORSTERI. [Jan. 4, 



to describe, and which are to be found not only on the inner face of 

 the operculum, but on that of the interoperculiim*. 



On the inner side of the former and projecting beyond its free 

 edge is a curved band of cartilage divided into several portions f. 

 One of these, that nearest the hyomandibular, is conical and bent at 

 an angle to that which follows it. The terminal plate is broad and 

 crescentic, and, on one side, was subdivided towards its free edge. 

 On the inner face of the posterior end of the interoperculum (I-op.) 

 there is an oval cartilage surrounded by eight or nine smaller 

 nodules. 



These cartilages represent the cartilaginous branchiostegal rays of 

 Plagiostomes, which are often subdivided into two groups — a dorsal 

 group attached to the hyomandibular, and a ventral group to the 

 cornual division of the hyoid arch. 



The prcEoperculum of Fishes and the squamosal of the higher Ver- 

 tebrata are represented by the bone Sq, termed "tympanic lamina" 

 by Dr. Giinther. I have marked the lower piece, which was distinct 

 on the specimen represented in fig. 4, Qu; but in another specimen 

 I can find no subdivision, and I am disposed to think that the divi- 

 sion arose from an accidental dismemberment of a squamosal (or 

 prseoperculum) corresponding with that of Menobranchus (P. Z. S. 

 1874, pi. xxix. fig. 1, S'^f), and that there is no true quadrate in 

 Ceratodus. I can discover no ossification of the substance of the 

 articular extremity of the suspensorium, such as occurs in the Am- 

 phibia. The pterygopalatines and the vomers, which last are repre- 

 sented only by the bases of the two vomerine teeth, are similar in their 

 form and relations to the corresponding bones of Urodele Amphibia. 



With respect to the branchial apparatus. Dr. Giinther (/. c. p. 520) 

 says that it " does not differ from that of Teleostean fish, but is 

 entirely cartilaginous. There are five branchial arches, the last 

 rudimentary and attached to the base of the fourth. There is no 

 peculiar modification of any part of this apparatus ; and the middle 

 pieces have the usual groove for the reception of the vessels and 

 nerves." 



I find that each of the anterior four branchial arches consists of a 

 long ventral and a short dorsal piece of cartilage : the former pieces 

 are united by ligament at their ventral ends, the third and fourth 

 curving forwards in this part; they are connected by joints sur- 

 rounded by ligaments with the dorsal pieces (fig. 2). In the median 



* Dr. Giinther terms the bone here named interoperculiim, " suboperculum." 

 He says {I. c. p. .'325) that to the lower edge of the operculum " is attached by 

 fibrous tissue the long styliform suboperculum, terminating at a considerable 

 distance from the mandibulary joint." It is quite true that the bone in ques- 

 tion is thus attached to the operculum ; but a much more important connexion 

 takes place between its ventral end and the angle of the jaw, by means of strong 

 ligainentous fibres which run into the hyomandibular ligament. It thus more 

 closely resembles the interoperculum of Teleosteans than it does the sub- 

 iiperculum. 



t In a specimen of the skull of Ceratodus, for which I am indebted to my 

 friend Mr. F. M. Balfour, the curved band of cartilage of the operculum is not 

 subdivided, and the interopercular cartilages are divided less regularly and 

 differentlvon the two sides. 



