I 78 THE AMERICAN NATURALIST. [VorL. XXXV. 
extended verbal description, and for this we must fall back on 
some modification of Steinmann's system, which for clearness 
and brevity can hardly be excelled. It is essential also that 
the student should not forget that the teeth are not, so to 
speak, individualities or specific organs, no matter how true 
they, in the main, are bred from generation to generation. 
Apart from their genetic origin as the selected results of 
the dynamic influence of external sculpture on limited areas 
of the shell (this sculpture itself being a selected conse- 
quence of the dynamic influence of the irregularities of the 
edge of the mantle upon the shell it secretes), the teeth are 
essentially prominences of a structure of homogeneous material 
modified by strains and impacts, and an excess or deficiency in 
the dynamics of the hinge margin in any individual will directly 
affect the hinge. The proof of this is most obvious in the 
differences of the lateral lamella which will be found in exam- 
ining a large series from different localities of such a type as 
Tellina, for instance. 
One important result of Bernard's researches was to show 
that cardinal and lateral are the disunited segments of an 
originally continuous lamina, and hence of genetically identical 
origin in certain teleodonts. In others the cardinals and 
laterals probably are distinct in origin (Tellina), though these 
developments so converge that in the adults they would be 
homologized by any student. The dynamic influence of the 
ligament when internal is very great and differs at different 
stages of growth in the same individual. In general it pre- 
vents the formation of the ** hook," from which cardinal teeth: 
are developed, on the laminz at the posterior end of the shell, 
but in some Leptonacea and Crassinella, where the internal 
resilium is nearly central prominences which are genetically 
identical with the so-called cardinals are formed also behind the 
beaks. The inclination of the teeth to a perpendicular from 
the beaks is a function of the torsion of the umbonal extremity 
of the valve and may differ strongly in otherwise closely related 
species. The cyclodont type of hinge largely results from such 
torsion. This inclination therefore should not be discussed as 
if it was an essential character of the teeth in themselves. 
