Vlll 
INTKODTTCTIOX. 
ralists are agreed upon the recognition of two main groups,—the 
one with the gill-clefts laterally placed, the other with these open¬ 
ings upon the ventral aspect. Willughby, in 1686, assigned the 
former to his “ Cartilaginei longi,” and termed the latter “ Cartila- 
ginei plani”; most subsequent writers have named the groups 
Squali (or “ Squales ”) and Rajas (or “ Eaies ’’) respectively; Dr. 
Giinther, as already remarked, employs the terms Selachoidei and 
Batoidei; and these divisions correspond more or less closely with 
those named Sharks and Bays in English. 
The palaeontological researches of Agassiz led Sir Bichard Owen 
to adopt a slightly different arrangement in 1860 1 , all the Bays 
forming one family (Baiidae), equivalent to another (Squalidae), 
comprising all recent Sharks except Cestracion ; while a third and 
fourth division, of equal rank, comprised respectively the extinct 
Hybodonts (Hybodontes), and the living Cestracion with its sup¬ 
posed extinct allies (Cestraciontidae). In 1866 2 the same author 
united the Hybodonts and Cestracionts into one suborder, named 
Cestraphori , in allusion to the presence of dorsal fin-spines; the 
Squalidae became the suborder of Selachii ; and the Baiidae formed 
a third suborder, the Batides. 
The researches of Prof. Carl Hasse upon the axial skeleton of the 
trunk led him, in 1882 3 , to propose another classification based 
upon the varied conditions of the notochordal sheath and the ver¬ 
tebrae ; and four groups were thus recognized. The Elasmobranchii 
diplosponclyli , or Palceonotidani, comprised the forms with a per¬ 
sistent notochord, typified by the existing Notidanus. The Cyclo- 
spondyli , represented by the Spinacidae, were defined as exhibiting 
a somewhat higher stage of specialization, the notochord being con¬ 
tracted at intervals by calcifications in the sheath, which constituted 
vertebrae in the shape of simple double-cones. The frequent addi¬ 
tion of concentric calcified rings outside this primitive double-cone 
was considered to justify the recognition of a third group, the 
Tectosponclyli ; and this comprised the modern Bays, with Pristio- 
phorus and Squatina. While a fourth division, that of the Astero- 
spondyli, was founded upon the Sharks, with vertebrae of an equally 
specialized type, but having the secondary peripheral calcifications 
so arranged as to appear radiating or star-shaped in vertical trans¬ 
verse section. 
1 R. Owen, Palaeontology, pp. 106-113. 
2 R. Owen, Anatomy of Vertebrates, vol. i. (1866), p. 13. 
3 C. Hasse, Das Naturliche System der Elasmobranchier. AUgemeiner Tkeil 
(1879), pp. 35-55. 
