IN THE PARAGUAYAN LEPIDOSIEEN, ETC. 343 



to the proximal portion of Meckel's cartilage, ultimately terminating at the point 

 where the latter makes its appearance on the outer surface of the ramus (fig. 7). There 

 is no representative in Lepidosiren of the bone which in Ceratodus has been identified 

 by Huxley [1. c.) as a dentary element. 



The hyoid arch consists of but a single element on each side — the equivalent of the 

 cerato-hyal of other Fishes. It is a somewhat curved, relatively thick bar, cylindrical 

 in the middle, laterally compressed proximally, but club-shaped inferiorly, where it is 

 somewhat loosely connected by ligament with its fellow (PI. XXVIII. figs. 1, 3, and 4, 

 c.h.). Dorsally, the proximal extremity of the cerato-hyal is connected with the skull 

 by a strong hyo-suspensorial ligament (figs. 3, 4, hy.s.L), which is partly inserted into 

 a rounded prominence on the inner surface of the suspensorial cartilage near its hinder 

 margin, and is partly continued as a broad band of fibres to the ventral surface of the 

 parasphenoid. A strong hyomandibular ligament also takes origin from the outer 

 surface of the proximal end of the cerato-hyal, and thence passes obliquely downward 

 to an insertion into the angle of the lower jaw. Structurally, the cerato-hyal consists 

 of an axial cartilaginous portion, invested, except at its proximal and distal extremities, 

 by a relatively thin sheath of superficial bone. There is no trace of any structures 

 corresponding to the hyomandibular, or basi- or glosso-hyal, cartilages of Ceratodus. 



A curious patch of cartilage is attached to the anterior surface of the distal portion 

 of the cerato-hyal, external to the osseous sheath, and quite distinct from the cartilage 

 of the distal extremity (PI. XXVIII. fig. 1). As to the significance of this cartilage I 

 can express no opinion beyond suggesting the possibility that it may be the remnant of 

 a cartilaginous hyoidean ray, and may therefore be included in the category of those 

 cartilages (vestigial hyoidean rays) which will be subsequently described in connection 

 with the opercular and interopercular bones. The existence of several similar patches 

 of cartilage in relation with the external surfaces of the ossified portions of the hyo- 

 mandibular and cerato-hyal elements of Acipenser has already been noticed by Parker 

 and Howes [32 a], by whom the cartilages in question were regarded as "rudimentary 

 branchial rays " [l. c. p. 174). The presence of such nodules or patches of cartilage 

 in connection with both hyoidean segments in this Ganoid affords a further resemblance 

 to the hyoidean rays of Elasmobranchs, and strengthens the above suggestion as to the 

 real nature of the still more reduced and vestigial rays of Lepidosiren. 



The opercular bones are represented in Lepidosiren by two elements — an operculum 

 and an interoperculum (" Kiemendeckelstiicke," Bischoff ; " Opercularknochen," Hyrtl ; 

 in Protopterus, " Kiemendeckelstucke," Peters ; " opercularia," Wiedersheim). The 

 operculum (in Protopterus, " preoperculum," Owen; "symplectic," Cobbold ; in 

 Ceratodus, " opercular," Giinther and Huxley) is a relatively slender bone, situated 

 immediately behind and parallel to the posterior margin of the squamosal (PI. XXVIII. 

 fig. 1, op.). Its inferior extremity forms a small disc-like base, and is closely attached 

 by ligament to the hinder border of the squamosal, a little dorsad to the articular 



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