INSTITUTE OF SCIENCE, PHILADELPHIA. 491 



In order to be effective as an obstacle to rotation in either direction, teetli 

 must alternate and there must be at least three surfaces opposed to one another. 

 That is to say, one tooth by the side of another projecting from the opposite 

 valve (formula^ J) might prevent L from rotating forward and R from rotat- 

 ing backward, but the converse rotations would not be provided against. With 

 a hinge arranged according to the formula [j, 1 ,, however, rotation would be op- 

 posed in either direction to an amount equivalent to the torsional strength of 

 the tooth R, or, if the two teeth of L were jointly less strong than the single 

 tooth of R, then their combined strength would measure the total resistance to 

 torsion. It is quite evident, therefore, that alternation is essential to the idea 

 of a perfectly efficient hinge, and must be expected wherever we find a fully 

 developed hinge. In this sense all hinges are Heterodont, and the distinction 

 drawn by Neumayr between Heterodont and Desmodont teeth, so far as their 

 situation is concerned, resolves itself into a comparison between teeth originally 

 laterals and both in one valve, with paired lamina: in the opposite valve, 

 on the one side; and teeth originally cardinals, or, if laterals, then laterals of 

 which one was in one valve and one in the other, with the clasping lamiiue 

 also alternated. The cases where laterals would both be in one valve and the 

 laminae all in the other would be exceptional, because such an arrangement in- 

 volves more or less asymmetry in form and weight of the valve which cannot 

 be regarded as a favorable condition for the animal, although it has prevailed, 

 to some extent, from the earliest ages. Such species are always relatively sed- 

 entary, and the asymmetry is usually proportioned to the immobility of the 

 particular species. An active form, if asymmetrical, would be constantly di- 

 verted from its intended line of motion by the mechanical effect, of the medium 

 in which it moved, unequally exerted upon the respective valves ; and this 

 would obviously be an unfavorable condition, leading to waste of energy and 

 impotency of volition. In the case of the hinge-teeth the asymmetry need not 

 be great and might be remedied by a compensating deposition of material on 

 the lighter valve, as occurs in Trigonia, but this is, as a matter of fact, very 

 exceptional. It would seem as if the tendency to torsional growth exemplified 

 in the spirality of Gastropods was originally anterior to the inception of the 

 Pelecypod type, as it is very common among the Paleoconchs (Antipleura, 

 Dualina, etc.) ; traceable in the sculpture of many Solenoconchs (Dentalnini) 

 and Pteropods ; and grows less prevalent with the progress of the Pelecypoda. 

 Among recent forms it is chiefly confined to representatives of the ancient 

 types (Prionodonta, such as Area, Stavelia and Unio) and is usually absent 

 among the Teleodonts, or most fully perfected modern forms. The asymmetry 

 of the siphonal end of such forms as Tellina, is of a different character from the 

 spiral twisting which involves the whole plane of the valves, as markedly illus- 

 trated by Antipleura. The latter is the tendency referred to in the preceding 

 remarks, though it is not impossible that asymmetry without torsion may be to 

 some extent connected with the torsional tendency. 



