36 ART. 4. — N. YATSTT : 



measuring 280 p on an average, does not increase in length 

 throughout all the larval stages. This edge which subserves the 

 function of a hinge for a long time even after the larvae forsake 

 the free-swimming life, I shall refer to as the hinge line — a term 

 which may at first appear inappropriate for the Ecardine Bra- 

 chiopods. At each extremity of the hinge line the shell is 

 bent slightly outwards, and forms the tooth, which is characteristic 

 of the Protegulum. Up to the time when these teeth are formed 

 the shell grows equally on all the radii of the semicircle, and 

 thus it results that on the hinge line which represents ap- 

 proximately a diameter, or the sum of two radii, the growth is 

 twice that of the height which corresponds to only a radius. A 

 comparison of the actual size of two larvae (PI. III., Fig. 53, 

 and PI. V., Fig. 74.) will best verify the above point : 



The ventral valve presents a greater curvature than the dorsal 

 (PI. IV., Fig. 71.). Both valves are still connected with each 

 other by means of thin cuticula, which, however, will soon be 

 torn. At the hinge line both valves increase in thickness (PI. 

 IV., Fig. 71.). 



The lobes of the mantle have become so thin that it can 

 hardly be seen except at its thickened margin, where there exist 

 the marginal lacuna and the glandular cells, becoming later the 

 glandular ridge (Drüsenwall) (PI. IV., Figs. 71, 72.). The mantle 



