A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

Geological Distribution of the Families of Elasmobranchs.

Pliocene	Miocene	Eocene	Cretaceous	Jurassic	Triassic	Permian	Coal Measures	Sub-Carbonifero	Devonian	Silurian	Ordovician
Cladoselachidæ									█	█
Acanthodii							█	█	█	█
Pleuracanthidæ							█	█	█	█
Cladodontidæ							█	█	█	█
Petalodontidæ							█	█	█
Psammodontidæ								█	█	█
Cochliodontidæ								█	█	█
Orodontidæ								█	█
Heterodontidæ	█	█	█	█	█	█	█
Tamiobatidæ										█
Hexanchidæ	█	█	█	█	█
Lamnidæ	█	█	█	█	█
Mitsukurinidæ	█	█	█	█
Odontaspidæ	█	█	█	█
Scyliorhinidæ	█	█	█	█	█
Carchariidæ	█	█	█	█
Squalidæ	█	█	█	█
Dalatiidæ	█	█	█
Squatinidæ	█	█	█	█	█
Rhinobatidæ	█	█	█	█	█
Pristididæ	█	█	█	█
Rajidæ	█	█	█	█
Narcobatidæ	█	█	█
Dasyatidæ	█	█	█	█
Myliobatidæ	█	█	█	█	█
Ptychodontidæ				█
Chimæridæ	█	█	█	█	█

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[CHAPTER XXV]
THE PROTOCHORDATA

The Chordate Animals.—Referring to our metaphor of the tree with its twigs as used in the chapter on classification we find the fishes with the higher vertebrates as parts of a great branch from which the lower twigs have mostly perished. This great branch, phylum, or line of descent is known in zoology as Chordata, and the organisms associated with it or composing it are chordate animals.

The chordate animals are those which at some stage of life possess a notochord or primitive dorsal cartilage which divides the interior of the body into two cavities. The dorsal cavity contains the great nerve centers or spinal cord; the ventral cavity contains the heart and alimentary canal. In all other animals which possess a body cavity, there is no division by a notochord, and the ganglia of the nervous system if existing are placed on the ventral side or in a ring about the mouth.