DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 359 



point of tlie present series, however, plates have begun to 

 develop, thus narrowing the pedicle-aperture, and rounding its 

 basal angles. With the growth of the plates more rapidly 

 along the lower portion of their inner edges, the foramen 

 shortens quickly, while narrowing but slowly, assuming in 

 the second stage (figure 16) a lanceolate, in the third stage 

 (figure 17) an oval, and in the fourth stage (figure 18) a 

 broadly circular outline. In the last two of these stages the 

 deltidial plates have come in contact with each other above 

 the apex of the dorsal valve, and the pedicle-aperture itself 

 has, from the second, if not from the first stage in the series, 

 encroached upon the apex of the valve, so that, as it attains 

 a circular outline, one-half its periphery is formed by the sub- 

 stance of the valve itself, and the other half by the deltidial 

 plates. From this stage upward there is no apparent change 

 in the actual dimensions of the foramen, and therefore with 

 the growth of the shell it becomes relatively much smaller. 

 It appears, however, that with the incurving of the cardinal 

 area and the concealment of the deltidial plates the foramen 

 becomes more and more enclosed by the apical portion of the 

 valve, and it may be that actual contact with the deltidial 

 plates in the last stage of development is lost. In this final 

 stadium, with the procumbent position of the ventral beak 

 upon the dorsal valve, the plane of the foramen is parallel to 

 the surface of the dorsal valve, and the aperture is therefore 

 lost to sight, or visible only at its upper edge. 



Plications. — Of the five and six plications visible upon the 

 youngest member of the series, three or four appear to exist 

 on that portion of the shell included within the earliest 

 growth-line, that is, presumptively, the initial shell, and they 

 increase by intercalation until, in the adult, the average 

 number is about sixty for each valve. Concentric lines of 

 growth follow each other with unusual rapidity, particularly 

 in early life. 



Summary. — Atrypa reticularis^ in the development of its 

 beak, foramen, and deltidial plates, is in essential harmony 

 with the other uniforaminate shells here discussed. The 



