DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 357 



geographically, in Paleozoic faunas, and is so familiar to 

 paleontologists, that a detailed description of its mature form 

 is here unnecessary. It is sufficient to remark that the pre- 

 vailing expression at this locality does not precisely conform 

 to the type of A. reticularis, but is more nearly that variety 

 described by Professor Hall (Pal. N. F., vol. ii, p. 271, 

 1852) under the name Atrypa rugosa. This is evident from 

 the development of the varical lamellae, which over the 

 plications are infolded into nearly tubular processes, some- 

 times produced at a strong angle from the shell to a length 

 of a millimetre or more. On the varices the plications are 

 covered by fine concentric wrinkles. The average size of 

 mature individuals, 25 x 25 mm., is less than that usual 

 to the species when occurring in later, especially Devonian, 

 faunas. 



Incipient Form (Plate XX, figures 12, 12 a, 15, 15 a). 

 The initial shell, or the actual inchoate period in its forma- 

 tion, is not at present known. The incipient shell of the 

 series here studied is very small, and can be but a few 

 removes from the initial stage. As just observed, it meas- 

 ures 2. 25 mm. in length by 3 mm. in width, and shows but 

 two concentric striae or growth-varices, with a correspond- 

 ingly slight development of the deltidial plates, so that this 

 shell may be regarded as but two stages advanced from the 

 actual inception of the shell. The test is flat, both valves 

 being shallow and depressed toward the anterior margin ; the 

 ventral beak high and erect, the dorsal beak inconspicuous 

 and rounded. The foramen, which is undoubtedly triangular 

 in the initial shell, has, at this stage, its basal angles slightly 

 rounded by the faintly developed deltidial plates. The pli- 

 cations are six in number on the ventral, and five on the 

 dorsal valve, the middle one of the latter not reaching as far 

 toward the beak as those adjoining it, and toward the ante- 

 rior margin being depressed below the lateral portions of the 

 shell. General outline sub-circular or sub-pentagonal, as in 

 the full-grown shell. 



