INTRODUCTION. 
IX 
In 1883, Prof. Gill* adopted the orders Squali (Sharks) and 
Eaj.® (Rays), and employed the results of Gegenbaur’s researches 
upon the skull in subdividing the latter, proposing to recognize 
four main groups. The Notidanidse, with a po.storbital articula- 
tion between the pterygo-quadrate and the cranium, were named 
Opistharthri ; the Cestraciontida;, with an antorbital articulation, 
the Proarihri ; the modern types of Sharks, the Anartliri ; and 
the Squatinidaj, the llhinae. In 1884, the first three divisions just 
named were also adopted by Prof. Cope* ; but the Bhince were 
now merged with the Anartliri. 
An examination of a large series of skulls and skeletons by Prof. 
Haswell, in 1884 also led him to diagnose great subdivisions by 
endoskeletal characters. The proposed arrangement, however, 
difiered but little from that of Dr. Gunther, the Selachoidei being 
only further subdivided into Pahroselachii (=Iiotidanida;) and 
Neoselaehii ( = other Sharks). 
About the same time, the discovery of Chlamydoselache h}’ ilr. 
Garman* 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in 1885’, this proposition was withdrawn, the new genus being 
placed with Cladodus in a division of the Galei. 
The discover}’ of Chlamydoselache, and the resemblance of its 
dentition to the fossil teeth named Diplodus also excited the interest 
of Prof. Cope, and led to the first attempt at a scientific description 
of a Palaeozoic Elasmobranch skull SufBcient materials had been 
obtained from the Permian beds of Texas to indicate that a fish 
possessing teeth of the i>//)fo<f««-type presented an arrangement of 
the mandibular and hyoid arches extremely similar to that observed 
in the living Botidanw ; 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 Ichthto- 
TOMi. This order was made to include the Hybodontidae, as de- 
I T. Gill, Bull. TJ. S. National Museum, no. 10 (1883), p. 967. 
^ E. D. Cope, Proo. Amer. Phil. Soe. 1884, p. 580. 
’ W. A. Haswell, Proc. Linn. Soc. N. S. Wales, vol. ix. (1884), pp. 71-119, 
pis. i. & ii. 
’ S. Garman, Science, vol. iii. (1884), p. 117. 
’ S. Garman, Bull. Mus. Comp. Zoology Harvard Coll. vol. lii. no. 1 (1885), 
p. 30. 
' E. D. Cope, Proc. Amer. Phil. Soc. 1884, pp. .')72-51l0, with plate. 
