BRACHIOPODA. 
69 
Some interesting observations have been made upon the development of the 
brachidium in Meristim Maria, which may be introduced in this place; it is 
probable that the facts observed are equally true of all the athyroids.* 
In the youngest condition of growth at which the spirals have been demon¬ 
strated (a shell with a length of 7 mm.) the ribbon makes six volutions (see 
Plate XLI, figure 9). The primary lamellae are far stronger than the remain¬ 
der of the ribbon, indeed it is often only these that can be detected, the rest of 
the coil being exceedingly delicate and leaving but a linear trace in the calcare¬ 
ous preparations. The cones are very depressed, in fact are coiled almost in 
the plane of the first volution, and their apices are far back of the transverse 
axis of the shell, which is not the case in the adult, where they lie in, or slightly 
in front of this axis. The second volution of the ribbon is scarcely more than 
one-half as long as the first, and the third stands in the same relation to the 
second. In these early stages of growth it has been impossible to determine 
fully the condition of the loop; it appears to be without the terminal bifurca¬ 
tion, and it is possible that this character was still undeveloped, though its 
absence may be accidental. In a later growth-stage (see Plate XLI, figure 10) 
the ribbon has greatly increased the number of its volutions, and the apices of 
the cones are more nearly central, but the cones themselves are still greatly 
depressed. The process of change from the primary condition of the spirals to 
their adult character was undoubtedly a complicated one, involving the con¬ 
stant resorption of the calcareous depositions made during the earlier stages. 
Attention may be directed to the effects of an accidental lesion or obstruction 
to the normal growth of this shell and its effect upon one of the spirals; the 
ribbon has adjusted itself with nicety, and probably without the disturbance of 
function to the contracted and irregular cavity of the valves. 
* Observations of similar import have been made upon the development of the spirals in Rhyncliospira 
evax. Hall; see Beecher and Clarke, Memoirs of the Wew York State Museum, vol. i. No. 1, p. 60. 
1889. 
