The present state of icthyological science recognizes eliminations that have been made from its main body. Comprehensively, a fish is a cold-blooded vertebrate, adapted for life in the water, breathing by means of gills, having the limbs, if present, in the form of fins, the smaller members being represented by cartilaginous rays connected by membrane. One or more fins are developed on the median line of the body.
The lancelets, myzonts, myxinoids, hag fishes, lampreys, sharks, and rays are recognized as differing sufficiently from the true fishes to entitle them to places of class distinction.
THE FRILLED SHARK—THE OLDEST LIVING TYPE OF VERTEBRATES.
The true fishes form one class; the elasmobranchs, sharks, and rays, another class; the marsipobranchs, myxinoid fishes, hag fishes, and lampreys, a class; and the lancelets and cirrostomes, a class. It will be seen, then, that technically there are four classes of fish-like vertebrates, where but one—fishes—has heretofore been recognized. The lancelets, as is well known, are the lowest in the scale, their structure being extremely simple. The skull in this class is undeveloped, the brain not distinctly differentiated, nor is there any heart.
The term Leptocardii, which designates this class, means thin heart, in reference to the simplicity of this portion of the arterial system.
At first sight of the mouth of the frilled shark, which is figured here, the teeth have a singular and wholly unnatural appearance, appearing like indented, leaf-like organs; but it is seen that there are three fangs, serpent-like, on a base, and several rows of them give the peculiar appearance, arranged as they are consecutively from before inward.
The Port Jackson sharks, of the family Heterodontidæ, have long been regarded as of great interest to paleontologists, from their being closely related to some extinct sharks. Under the term Cestracion (now Gyropleurodus), these sharks are known to naturalists. A species, G. francisci, is now found off the coast of California.
Cestracion phillipi is found in the Australian seas. The term cestracion is from the Greek kestra, a weapon. Many of the extinct species are known by the preservation of this spine, which being of more durable structure is preserved after all other traces of the creature have passed away.
