1887.] PROF. T. J. PARKER ON CARCHARODON RONDELETII. 33 



in so large a specimen a small cartilage imbedded in the immense 

 jaw-nmscles would be easily missed. 



The foramina in the sivull-wall have the same general disposition 

 as in Lamna (cf. figs. 5 and 11, Plate V.), the main differences 

 between the two being the greater proportional size of the optic 

 foramen (ii.) in the latter genus, and the fact that the oculomotor 

 foramen (Hi.) is on the same level as the o])tic in Carcharodon, 

 wliile in Lamna it is in the same horizontal plane as the ophthalmic 

 peduncle (op.ped.). In both tlie carotid foramen (cor.f.) is a short 

 distance caudad of the optic. Between and below the foramina 

 for the 3rd (m.) and the 5th {v.) nerves there is, in Lamna, a small 

 aperture wliich does not seem to be represented in Carcharodon : 

 possibly it transmits the 6th nerve. The glossopharyngeal and the 

 vagus foramina (Plate IV. figs. I and 2, ix., x.) are both large, the 

 latter in particular being of immense size. 



The jaws of Carcharodon (fig. 5) are chiefly remarkable for their 

 great size, and esf)ecially for tlie extraordinary depth of tlie mandible. 

 In Lamna (fig. 1 1) their proportional size is considerably less. 



In another closely allied genus. Alopecias, the cranium has a more 

 rounded form than in Lamna, and is similarly modified in accordance 

 with the great size of the eyes. The rostrum is very thin and 

 delicate, and is hardly at all calcified : its ventral or prenasal bar is 

 perforated at its distal end by a vertical foramen. The jaws have 

 about the same proportional size as in Lamna. 



The gill-bearing arches of Carcharodon closely resemble those of 

 Lamna and of Scyllium '. The hyomandibnlar and ceratohyal 

 ( Plate y III. fig. 27, c.hy) are large and stout, and the tongue is sup- 

 ported by a flat basihyal {b.hy) having a convex anterior and an ex- 

 cavated posterior border. The first branchial arch consists of aflat, 

 subtriangular pharyngohyal, a stout epibranchifd, and a similar but 

 longer ceratobranchial {c.br. 1) which articulates with the basi- 

 hyal, there being no first hypobranchial. The next three arches 

 have, in addition, a short rod-like hypobranchial segment {h.br. 2-4 ). 

 Between the ventral or inner ends of tiie second hypobranchials 

 (Ji.br. 2) is a small nodular basibranchial {b.br. 2), The second 

 and third hypobranchials are subequal, the fourth (Ji.br. 4) is 

 barely half the length of its predecessors. The fourth and fifth 

 pharyngobranchials have undergone concrescence ; the fifth cerato- 

 branchial {c.br. 5) is, as usual, much larger than the corresponding 

 segment in the preceding arches. The last arch has no hypo- 

 branchial, its ceratobranchial segment {c.br. 5) abutting against 

 an elongated flattened plate {b.br. 5), rounded in front and pointed 

 behind, and probably to be regarded as a fifth basibranchial. 



To the hiner face of the fifth ceratobranchial, near its dorsal end, 

 a small irregular rod of cartilage is attached by fibrous tissue. Can 

 this be the rudiment of a sixth branchial arch 1 



The gill-arches are but slightly calcified, even the hyomandibnlar 

 and ceratohyal having only a thin crust of bony matter which does 

 not extend to their extremities. 



1 W. K. Parker, op. cit. ; Gegenbaur, ' Kopfskelet der Selachier,' Ijeipzig, 1872, 

 Proc. Zool. Soc— 1887, No. III. 3 



