252 STUDIES IN EVOLUTION 



therefore, is not primarily a line of articulation of the valves, 

 but the limiting borders between the body and the attached 

 edges of the mantle. Secondarily, and during later growth, 

 the extension of the valves along a line of apposition forms a 

 true hinge-line. 



The first points of contact of the valves to form the true 

 hinge lie adjacent to the right and left sides of the body of 

 the animal, at the cardinal extremities (figure 99, t). Here 

 naturally the first hinge-teeth are formed, and their position 

 corresponds to that in adult individuals; namely, on each 

 side of the cardinal opening. The enlarging of the cardinal 

 opening by shell growth results in the gradual divergence 

 or separation of the teeth, as in Terebratulina. In species 

 with extended hinge-lines, as in many forms of Spirifer, 

 Orthis, and Strophometia, the teeth still lie in their original 

 position on each side of the cardinal opening, and the elon- 

 gation of the hinge has come not only from the enlargement 

 of the opening by growth, but by additions at the hinge 

 extremities, so that the teeth are situated on each side of the 

 central area, below the beak, and not at the cardinal angles. 

 The young of these genera, however, all have the hinge-teeth 

 at the extremities of the hinge, as the cardinal opening then 

 occupies the whole posterior area of the shell. 



Adult specimens of Katorgina (/f, cingulata Billings) have 

 a deltidium as in Strophomena. The cardinal opening in- 

 cluding the deltidium occupies the whole posterior end of 

 the shell, and according to a statement made to the writer 

 by Mr. Charles Schuchert, there are rudimentary teeth at 

 the cardinal extremities. Therefore this genus represents a 

 nepionic condition of later forms, and, on account of these 

 and other characters, it is believed to be related to Orthisina 

 and Strophomena^ of which it is the ancestral type. It con- 

 sequently belongs to the articulate brachiopods. 



The embryonic stages up to this point have frequently 

 been compared to similar stages in other organisms, especially 

 in the Annelida and Polj-zoa. Without repeating these com- 



