Holocephali Chimceroidei. 93 



fossils, those named Carcharodon megalodon (Fig. 127) having an 

 almost world-wide distribution. Specimens are exhibited from 

 New Zealand, Australia, South Carolina, the West Indies, 

 France, Spain, Italy, Malta, and Arabia, as also from the 

 Antwerp and Suffolk Crags. The Carchariidee are almost, if 

 not exclusively, Tertiary, and only a small collection of teeth of 

 Galeocerdo, Carcharias, Hemipristis, etc., is exhibited. 



It may be interesting to add that in some places, both in the 

 Atlantic and Pacific (especially at extreme depths in the red-clay 

 areas), the " Challenger " dredged up many teeth of Sharks and 

 ear-bones of Whales, all in a semi-fossil state, and usually im- 

 pregnated with oxides of iron and manganese. The Sharks' 

 teeth belong principally to species believed to be extinct, and 

 resemble those found fossil in the late Tertiary formations. 



Sub-class II. HOLOCEPHALI. 



ORDER I. Chimseroidei. 



The Chimoeras resemble the Sharks in many important Wail case 

 features, but, in the skull, the upper jaw is fused with the NO. 3, and 

 cranial cartilage, not suspended by the upper part of the hyoid Table- case, 

 arch. The skeleton is wholly cartilaginous, and the notochord No ' 33 " 

 is tolerably persistent, the vertebrae being represented by mere 

 slender rings. In the two living genera, there is a strong- 

 spine in front of the dorsal fin : the gill-clefts are covered by a 

 fold of skin, so that only a single external opening is observed : 

 and the dentition consists of four plates above and two below. 



Teeth of Rhynchodus and Palceomylns from the Devonian of 

 North America, and of Ptyctodus from the Devonian of Russia, 

 are the earliest fossils hitherto definitely referred to this sub- 

 class, but there are no examples in the Collection. The early 

 Jurassic family of Squaloraiida3 is represented by the unique 

 genus Squaloraja, of which several fine specimens are exhibited 

 from the Lower Lias of Lyme Regis. The trunk is shaped 

 like that of a narrow skate of the family Rhinobatidas, and the 

 rostral spine in the male is long and slender ; there is no dorsal 

 fin-spine. Another Jurassic family is that of the Myriacan- 

 thidae. Myriacanthus itself, also from the Lower Lias of Lyme 

 Regis, is represented by numerous fragmentary specimens. 

 One slab of M. granulatus shows the dorsal and rostral spines, 

 and a produced snout resembling that of the existing Callo- 

 rhynchus. The dentition (originally named Prognathodus) is 

 remarkable for a median chisel-like tooth in front of the lower 

 jaw. The long dorsal spine is covered with tubercles, which 

 are often pointed and thorn-shaped. The still-surviving family 

 of Chimseridse is first represented by teeth of Ganodus and 



