In the same line of study is Dr. R. T. Jackson's[261] work on the mechanical origin of characters in the lamellibranch molluscs. "The bivalve nature of the shell doubtless arose," he says, "from the splitting on the median line of a primitive univalvular ancestor;" and he adds: "A parallel case is seen in the development of a bivalve shell in ancient crustaceans;" in both types of shells "the form is induced by the mechanical conditions of the case." The adductor muscles of bivalve molluscs and crustaceans are, he shows plainly, the necessary consequence of the bivalvular condition. In his theory as to the origin of the siphon of the clam (_Mya arenaria_), he explains it in a manner identical with Lamarck's explanations of the origin of the wading and swimming birds, etc., even to the use of the words "effort" and "habit." "In _Mya arenaria_ we find a highly elongated siphon. In the young the siphon hardly extends beyond the borders of the valves, and then the animal lives at or close to the surface. In progressive growth, as the animal burrows deeper, the siphon elongates, until it attains a length many times the total length of the valves. "The ontogeny of the individual and the paleontology of the family both show that Mya came from a form with a very abbreviated siphon, and it seems evident that the long siphon of this genus was brought about by the effort to reach the surface induced by the habit of deep burial." "The tendency to equalize the form of growth in a horizontal plane, or the geomalic tendency of Professor Hyatt,[262] is seen markedly in pelecypods. In forms which crawl on the free borders of the valves, the right and left growth in relation to the perpendicular is obvious, and agrees with the right and left sides of the animal. In Pecten the animal at rest lies on the right valve, and swims or flies with the right valve lowermost. Here equalization to the right and left of the perpendicular line passing through the centre of gravity is very marked (especially in the Vola division of the group); but the induced right and left aspect corresponds to the dorsal and ventral sides of the animal, not the right and left sides, as in the former case. Lima, a near ally of Pecten, swims with the edges of the valves perpendicular. In this case the geomalic growth corresponds to the right and left sides of the animal.