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No fossil shells have yet been found in the deposit, nor plants in a determinable state. The fossil fish are numerous and very perfect; but they are of a peculiar type, called Ischypterus, by Sir Philip Egerton, from the great size and strength of the fulcral rays of the dorsal fin, from ischus, strength, and pteron, a fin.
Its distinguishing feature is a shield or disc extending from the tip of the upper jaw to a point behind the shoulders, and said to be a modification of the spurious dorsal fin. This structure consists of a midrib and a number of transverse flat ridges capable of being raised or depressed. The disc has a membranous continuous edge or margin.
Their hearts leapt in their breasts at this startling news, and, looking hurriedly round, they perceived, to their horror, that several black triangular dorsal fins were cleaving the water in their wake, and closing rapidly in upon them. Fortunately the water in the direction in which they were swimming was as yet clear, to all appearance. "Cannot you send a boat, sir?
Each vertebra consists of a body, the solid central portion, and a number of projections called processes. Those which spring from the posterior of each arch are the spinous processes. In the dorsal region they are plainly seen and felt in thin persons. The bones of the spinal column are arranged in three slight and graceful curves.
The pleural sacs that enclose the lungs are dorsal parts of the head-coelom, originating from the pleuroducts; the pericardium in which the heart afterwards lies is also double originally, being formed from ventral halves of the head-coelom, which only combine at a later stage.
Twemlow began to make the introductions a long and tedious process, which she performed relentlessly, without haste and without scamping her work. With each member of the aristocracy of his new profession Ashe shook hands, and on each member he smiled, until his facial and dorsal muscles were like to crack under the strain.
"If you mean pronated in relation to the arm, that is impossible, for the position of the egg-patches shows clearly that the bones of the arm were lying in the position of supination. Thus the dorsal surface of the arm and the palmar surface of the hand respectively were uppermost, which is an anatomical impossibility so long as the hand is attached to the arm."
The size of their dorsal fins, which were carried well out of the water, assured us that our followers were sharks of the largest size, and enabled us to form a pretty fair idea of what would be our fate should any of us be unfortunate enough to fall or be washed off the raft.
The tail, of very variable form, always bears THREE pairs of setae at its hinder margin. The Zoeae of the Crabs usually maintain themselves in the water in such a manner that the dorsal spine stands upwards, the abdomen is bent forwards, the inner branch of the natatory feet is directed forwards, and the outer one outwards and upwards. Tails of the Zoeae of various Crabs. Pinnotheres. Sesarma.
It would seem far more natural to derive the vertebrate from some free swimming form like the schematic worm, whose largest nerve-cord lay on the dorsal surface because its branches ran to heavy muscles much used in swimming. Later the other nerve-cords degenerated, for such a degeneration of nerve-cords is not at all impossible or improbable.