INTRODUCTION. 
IX 
In 1883, Prof. Gill 1 adopted the orders Squali (Sharks) and 
Raj^e (Pays), and employed the results of Gegenbaur’s researches 
upon the skull in subdividing the latter, proposing to recognize 
- four main groups. The Notidanidae, with a postorbital articula¬ 
tion between the pterygo-quadrate and the cranium, weie named 
Opistharthri ; the Cestraciontidse, with an antorbital articulation, 
the Proarthri ; the modern types of Sharks, the Anarthri ; and 
the Squatinidae, the Rhince. In 1884, the first three divisions just 
named were also adopted by Prof. Cope 2 ; but the Rhince were 
now merged with the Anarthri. 
An examination of a large series of skulls and skeletons by Prof. 
Haswell, in 1884 3 , also led him to diagnose great subdivisions by 
endoskeletal characters. The proposed arrangement, however, 
differed but little from that of Dr. Gunther, the Selachoidei being 
only further subdivided into Palceoselachii (=Notidanidae) and 
Neoselachii ( = other Sharks). 
About the same time, the discovery of Chlamydoselache by Mr. 
Garman 4 induced him to add to the orders Galei ( = Selachoidei) 
and Batoidei, a supposed new order, Selachophichthyoidei, charac¬ 
terized by “ vertebrae partially or imperfectly developed, a persistent 
notochord, and teeth with broad backward-expanded bases;” but 
in 1885 5 , this proposition was withdrawn, the new genus being 
placed with Cladodus in a division of the Galei. 
The discovery of Chlamydoselache , and the resemblance of its 
dentition to the fossil teeth named Dvplodus also excited the interest 
of Prof. Cope, and led to the first attempt at a scientific description 
of a Palaeozoic Elasmobranch skull 6 . Sufficient materials had been 
obtained from the Permian beds of Texas to indicate that a fish 
possessing teeth of the Diploclus-type presented an arrangement of 
the mandibular and hyoid arches extremely similar to that observed 
in the living Notidanus ; and the supposed presence not only of a 
few definite tracts of ossification in the chondrocranium, but also of 
imperfect membrane-bones, was considered to justify the recognition 
of a new order of the Elasmobranch subclass, to be termed Ichthyo- 
tomi. This order was made to include the Hybodontidse, as de- 
1 T. Gill, Bull. TJ. S. National Museum, no. 16 (1883), p. 967. 
2 E. D. Cope, Proc. Amer. Phil. Soc. 1884, p. 580. 
3 W. A. Haswell, Proc. Linn. Soc. N. S. Wales, vol. ix. (1884), pp. 71-119, 
pis. i. & ii. 
4 S. Garman, Science, vol. iii. (1884), p. 117. 
5 S. Garman, Bull. Mus. Comp. ZoologT Harvard Coll. vol. xii. no. 1 (1885), 
p. 30. 
6 E. E>. Cope, Proc. Amer. Phil. Soc. 1884, pp. 572-590, with plate. 
