Fig. 351.—Skeleton of Chimæra monstrosa Linnæus. (After Dean.)
Family Chimæridæ.—The existing Chimæras are known also as spookfishes, ratfishes, and elephant-fishes. These are divided by Garman into three families, and in the principal family, the Chimæridæ, the snout is blunt, the skin without plates, and the dorsal fin is provided with a long spine. The flat tritors vary in the different genera. The single genus represented among living fishes is Chimæra, found in cold seas and in the oceanic depths. The best-known species, Chimæra colliei, the elephant-fish, or chimæra of California, abounds in shallow waters of ten to twenty fathoms from Sitka to San Diego. It is a harmless fish, useless except for the oil in its liver, and of special interest to anatomists as the only member of the family to be found when desired for dissection. This species was first found at Monterey by Mr. Collie, naturalist of Captain Beechey's ship, the Blossom. It is brown in color, with whitish spots, and reaches a length of 2½ feet. As a shallow-water form, with certain differences in the claspers and in the tail, Chimæra colliei is sometimes placed in a distinct genus, Hydrolagus. Other species inhabit much greater depths and have the tail produced into a long filament. Of these, Chimæra monstrosa, the sea-cat of the north Atlantic, has been longer known than any other Chimæra. Chimæra affinis has been dredged in the Gulf Stream and off Portugal. Chimæra phantasma and Chimæra mitsukurii are frequently taken in Japan, and the huge jet-black Chimæra purpurascens in Hawaii and Japan. None of these species are valued as food, but all impress the spectator with their curious forms.
Fig. 352.—Elephant-fish, Chimæra colliei Lay & Bennett. Monterey.
The fossil Chimæridæ, although numerous from Triassic times and referred to several genera, are known chiefly by their teeth with occasional fin-spines, frontal holders, or impressions of parts of the skeleton. The earliest of chimæroid remains has been described by Dr. Charles D. Walcott[151] from Ordovician or Lower Silurian rocks at Cañon City, Colorado. Of the species called Dictyorhabdus priscus, only parts supposed to be the sheath of the notochord have been preserved. Dr. Dean thinks this more likely to be part of the axis of a cephalopod shell. The definitely known Chimæridæ are mainly confined to the rocks of the Mesozoic and subsequent eras. Ischyodus priscus (avitus) of the lower Jura resembles a modern chimæra. Granodus oweni is another extinct chimæra, and numerous fin-spines, teeth, and other fragments in the Cretaceous and Eocene of America and Europe are referred to Edaphodon. A species of Chimæra has been recorded from the Pliocene of Tuscany, and one of Callorhynchus from the greensand of New Zealand. Other American Cretaceous genera of chimæroids are Mylognathus, Bryactinus, Isotænia, Leptomylus, and Sphagepœa. Dental plates called Rhynchodus are found in the Devonian.
Rhinochimæridæ.—The most degenerate of existing chimæras belong to the family of Rhinochimæridæ, characterized by the long flat soft blade in which the snout terminates. This structure resembles that seen in the deep-sea shark, Mitsukurina, and in Polyodon. In Rhinochimæra pacifica of Japan the teeth in each jaw form but a single plate. In Harriotta raleighana, of the Gulf Stream, they are more nearly as in Chimæra. Both are bathybial fishes, soft in texture, and found in great depths. The family of Callorhynchidæ, or Antarctic Chimæras, includes the bottle-nosed Chimæra (Callorhynchus callorhynchus) of the Patagonian region. In this species the snout is also produced, a portion being turned backward below in front of the mouth, forming a sensory pad well supplied with nerves.
Extinct Chimæroids.—According to Woodward, three other families are recognizable among the extinct forms.
The Ptyctodontidæ are known from the teeth only, a single pair of large, laterally compressed dental plates in each jaw, with a few hard tritoral areas. These occur in Silurian and Devonian rocks. Ptyctodus obliquus from the Devonian of Russia is the best-known species. Other genera are Rhynchodus and Palæomylus.
The Squalorajidæ have the head depressed and the snout produced in a flat rostrum, as in Harriotta. There is no dorsal spine, and the teeth are a few thin curved plates. The frontal holder of the male is well developed. The few species occur in the Lias. Squaloraja dolichognathos is known from numerous fragments from the Triassic in England and Scotland. Chalcodus permianus is found in German Permian.
The Myriacanthidæ have the body elongate, with dermal plates on the head and a long straight spine in the dorsal fin. The frontal holder is large. The species, few in number, are found in Mesozoic rocks. Myriacanthus paradoxus is the best-known species. Of another species, Chimæropsis paradoxa, a skeleton about three feet long has been found which shows a number of peculiar traits. The skin is covered with ribbed shagreen scales. The dorsal fin has a large spine with retrorse serrations behind. The tail is slim, and the pectoral and ventral fins are very large. Bony plates with conical spines protect the neck. The teeth are large and angular, of peculiar form.