SILURIAN BRACHIOPODA 51 



the foramen (plate 4, fig. 18). A specimen 5 mm. in length shows the still 

 further increase in the size of these plates, although they do not come in 

 contact, but leave an oval opening extending from the ventral beak down 

 to the beak of the dorsal valve (plate 4, fig. 19). The increase in the 

 growth of the deltidial plates along their inner margins brings them in con- 

 tact under the dorsal beak, in specimens having a length of about 7 mm. 

 (plate 4, fig. 20). Further growth truncates their inner angles, thus short- 

 ening the deltidial opening. In individuals about 12 mm. long (fig. 21), 

 the opening extends but little more than half the length of the area, and 

 the lower margin of the opening is thickened and slightly deflected. Fully 

 matured forms, having a length of from 15 to 17 mm., have a perforation 

 less than one-half the height of the area, which truncates the beak more 

 strongly than in younger shells, and the deltidial plates show a defined 

 thickened area below the perforation, often extending to the dorsal beak 

 (plate 4, fig. 22). 



Variations. As already stated, the elongate and broadly flabellate 

 shells appear to be, in this species, neither common nor genetic variations. 

 Among the extraordinary developments are specimens with duplicate pli- 

 cations in the sinus, and one showing but seven plications on the shell. 

 Another individual has the initial shell strongly defined by a varix of 

 growth, and shows on this portion ten plications, but in the subsequent 

 growth, only eight plications are continued, these alternating at the varix 

 with those of the embryonic shell. 



Atrvpa reticularis, Linnaeus, 1767 



Plate VI, Figs. 12-20 



, Hall. Twenty-fourth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 162. 1879. 



, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 304. 1882. 



The abundance of this well-known species at Waldron has afforded the 

 means of studying its developmental stages with very satisfactory results. 

 All the individuals, from the earliest observed stage upward, agree in con- 



