Many living genera of Euselachii are of considerable antiquity; all the recent families except the Car chariidae and Trygonidae have genera living to-day that are found fossil in Cretaceous strata, and such specialized forms as Squatina and Rhinobatus occur in the Jurassic, together with Heterodontus, Pristiurus and Hexanchus. The curious Scapano rhynchus, with long shovel-like snout, was discovered in 1898 living in deep water off Japan, but had long been known from Cretaceous rocks. Protospinax, a recently described Jurassic shark, appears to stand in the same relationship to Pristiophorus that Rhinobatus does to Pristis. The saw-shark (Pristiophorus) has the snout produced into a long blade with a series of teeth on each side, and in this resembles the saw-ray (Pristis) ; but Pristis is evidently a modified Rhinobatus, and the discovery of Pro tospinax (Woodward, 1918), a shark with the snout produced and flattened as in Rhinobatia, but except for the more primitive median fins otherwise similar to Pristiophorus, is of some interest.
The Euselachii are not certainly known before the Trias, for the Carboniferous and Permian Orodontidae, Cochiliodontidae, Petalodontidae and Psammodontidae, generally assigned to this sub-class, are mainly known from teeth. The Holocephali also date back to the Trias. Of the Palaeozoic sub-classes only the Pleuropterygii can be regarded as ancestral, the Devonian Cladoselachus being the most primitive shark known.
The researches of Stensio (1925) have re vealed the relationship to the Selachii of the Arthrodira, a group of Palaeozoic fish-like vertebrates in which the head is covered by a shield of bony plates, movably articulated with a bony cara pace that protects the anterior part of the body. Below the
head-shield Stensio has found a cartilaginous cranium wholly or partly invested inside and out by a thin layer of bone; this cran ium is of typical Selachian form, with prominent olfactory cap sules; there are no dermal bones (parasphenoid, vomers) below it. It is clear that the Arthrodira are not Pisces, but are an in dependent offshoot of the Selachii. It is, perhaps, best to place them in a separate class, Placodermi, which will include also the similarly armoured Palaeozoic Asterolepida (Pterichthys, etc.).
all with good bibliographies: T. W. Bridge, Cambridge Natural History, Fishes (1904) ; T. F. Daniel, The Elasmo branch Fishes, Univ. of California (1928) ; E. S. Goodrich, Lankester's Zoology, Fishes (1909) ; C. T. Regan, "Classification of Selachians," Proc. Zool. Soc. (1906) ; Systematic Monograph of Euselachii; S. Gar man, The Plagiostomia, Mem. Mus. Comp. Zool. 36 (1913) ; Palaeon tology: E. A. Stensio, The Head of the Macropetalichthyids (Arthro dira), Field Mus. Publ. Geol. 4, No. 4 (5925) ; A. S. Woodward, Cat. Fossil Fishes, z and 2, Brit. Mus. (1889, 1891) ; New Jurassic Elasmo branchs, Proc. Zool. Soc. (1918) ; Holocephali; B. Dean, Chimaeroid Fishes and their Development, Carnegie Inst. (1906) ; S. Garman, The Chismopnea (Chimaeroids) (systematic monograph) : Mem. Mus. Comp.
Harvard, 4o, No. 3 (191I) ; Fins: R. C. Osburn, "Origin of the paired Limbs of Vertebrates," Amer. fount. Anat. 7 k1907).
(C. T. R.)