BRACHIOPODA. 69 



Some interesting observations have been made upon the development of the 

 brachidium in Meristina 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. 



* Obsen-ations of similar impoi-t bave been made upon the development of the Bpirals in Bhynehosplra 

 max. Hall ; see Bbbchbh ami Clarkb, Memoirs of the INew York State Museum, vol. i, No. 1, p. 60. 

 1889. 



