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Show 36 PROF. T. H. HUXLEY ON CERATODUS 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 interoperculum*. 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 itsjree edge. On the inner face of the posterior end of the interoperculum (Lop.) 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 preeoperculum of Fishes and the squamosal of the higher Vertebrata are represented by the bone Sq, termed "tympanic lamina" by Dr. Gunther. 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 division arose from an accidental dismemberment of a squamosal (or praeoperculum) corresponding with that of Menobranchus (P. Z. S. 1874, pl. xxix. fig. 1, Sq), 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 A m phibia. The pterygopalatines and the vomers, which last are represented 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. Gunther (I. c. p. 526) 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 surrounded by ligaments with the dorsal pieces (fig. 2). In the median * Dr. Gunther terms the bone here named interoperculum, " suboperculum." H e says (I. c. p. 525) 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 question 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 ligamentous fibres which run into the hyomandibular ligament. It thus more closely resembles the interoperculum of Teleosteans than it does the suboperculum. t In a specimen of the skull of Ceratodus, for which I a m indebted to m y 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 differently on the two sides. |
