18S C. E. Beecher — Development of the Brachiopoda. 



binge line of the future valves, hi, figures 26, 27. Thus its 

 origin is not, as in pelecypods, a line produced by the bending 

 of a single plate (Jackson), but is the line along which the two 

 mantle lobes are bent against the body. Between them pro- 

 jects posteriorly nearly half the body of the animal, and the 

 whole opening corresponds to the pedicle opening of later 

 stages of growth. The hinge of brachiopods, 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 15, 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 Strophomena, the teeth still lie in their original position 

 on each side of the cardinal opening, and the elongation 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 Kutorgina {K. 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 con- 

 dition of later forms, and, on account of these and other 

 characters, it is believed to be related to Orthisina and Stro- 

 phomena, of which it is the ancestral type. It consequently 

 belongs to the articulate brachiopods. 



The embryonic stages up to this point have frecruently been 

 compared to similar stages in other organisms, especially in 

 the Annelida and Polyzoa. Without repeating these com- 

 parisons, which may be consulted elsewhere, 4 > 12 ' 15 ' 16 ~ 19 ' 21 atten- 

 tion is called to the similarity of development of the brachiopod 

 typembryo to the larval stages of Spirorbis. There are, how- 



