32 PROF. T. J. PARKER ON CARCHARODON RONDELETII. [Jail. 18, 



of comparing the skull with that of the closely allied Porbeagle 

 {Lamna cornubica), I give upper, under, and side views of the cranium 

 of both Sharks, drawn to the same absolute length ; in the side 

 views the jaws also are shown (Plates IV. and V.). 



Haswell says, " On comparing the skull of Carcharodon with a 

 dried skull of Lamna cornubica, T can find little difference between 

 the two." As a matter of fact the differences between the two 

 crania are by no means inconsiderable, as Haswell would no doubt 

 have found if his skull of Lamna had not been distorted by drying. 



The main differences are dependent upon the much greater pro- 

 portional size of the rostrum and of the orbits in Lamna. In 

 Carcharodon the dorso-lateral arms of the rostrum (first labial 

 cartilages, W. K. Parker^) are slightly curved, with a downward 

 concavity, while the median ventral arm (prenasal cartilage) has a 

 strong downward convexity, so that the three bars meet at a wide 

 angle. All three bars are broad at the base, but narrow considerably 

 in front, and are but slightly calcified, the distal portion of the 

 entire rostrum being composed wholly of cartilage. 



In Lamna^ on the contrary, all three rostral bars have a marked 

 sigmoid curve, and meet with one another at very acute angles. 

 They are also much longer proportionally than in Carcharodon, 

 much thicker, and are covered externally with a close mosaic of 

 bony matter. 



As already remarked, the eyes, and consequently the orbits, are 

 proportionally much larger in Lamna than in Carcharodon ; as a 

 result of this, the orbital roofs {sup. orb. pi) in Lamna are strongly 

 arched both antero-posteriorly and laterally, and the infraorbital plates 

 {inf.orb.pl) inclined downwards at their outer ends. The whole 

 cranium also, and especially the basal plate {i. e. the basis cranii 

 proper plvs the infraorbital plates), is much narrower than in 

 Carcharodon {cf. figs. 3 and 4), and the parotic (p.ot.pr) and post- 

 orbital {p.orb.pr) processes are less prominent. 



In the fcetal specimen (Plate VIII. figs. 24 and 25) the rostrum is 

 very slender, and its ventral or prenasal bar is perforated distally by a 

 foramen. The anterior fontanelle {font.) is very large, and allows 

 the cerebrum to be partly seen in a view from above. The auditory 

 capsules are very prominent, and show clearly the elevations for the 

 semicircular canals. The supraorbital plate is hardly developed, 

 and the infraorbital plate is quite narrow. 



To the outer surface of the auditory capsule of the foetus, dorsad 

 of the hyomandibular facet, a small rod of cartilage (PI. VIII. fig. 26, 

 spir.cart.) is attached by fibrous tissue. This appears to be the 

 dorsal segment of the mandibular arch or spiracular cartilage. Unfor- 

 tunately the specimen had been dissected by one of my assistants, as 

 a help to the articulation of the adult skeleton, before I observed 

 this cartilage, so that I was unable to make out its relations to the 

 spiracle. No corresponding structure was found in the adult, but 



^ " On the Structure and Development of the Skull in Sharks and Eays," 

 Trans. Zool. Soc. vol. x. (1877) p. 189. Parker and Bettany, ' Morphology of 

 the Skull,' p. 35. 



