46 MEMOIRS OF THE STATE MUSEUM 



sinus and fold, respectively, marks the feature as an interesting one, to 

 which we call attention more at length in the description of the species 

 Rhynchotreta cuneata and Atrypa reticularis. Rare instances occur of 

 individuals assuming all the characters of maturity before attaining a 

 length of 6 mm., and from this point upward to the normal size for adult 

 growth, mature dwarfs are frequently found. 



Beak. In the initial shell, the beak of the dorsal valve is rounded and 

 inconspicuous, and so remains in all stages of growth. In the opposite 

 valve, the beak is at first high, erect but not acute, the cardinal margins 

 sloping abruptly, and with increasing age the beak becomes fuller, more and 

 more incurved at the apex, but is never closely procumbent upon the dorsal 

 umbo, as is the case in maturity with most of the plicate species here 

 described. 



Foramen. At the outset the pedicle-aperture is narrowly subtrian- 

 gular, reaching to and encroaching upon the apex, free of deltidial plates 

 and with the lateral margins unthickened, i. e., elemental in every respect. 

 In the second stage of growth (after the appearance of plications on the 

 surface, dimensions 1.5x1.1 mm.), the apertural margins have become 

 thickened, and directly thereafter, the deltidial plates begin to develop, 

 gradually narrowing the aperture at the base. The symphysis of these 

 plates with the valve is marked by distinctly elevated lines. In maturity, 

 the deltidial plates have developed sufficiently to completely close the lower 

 part of the aperture, coming together behind the beak of the dorsal valve, 

 and giving to the foramen an elliptical outline constricted toward the apex, 

 where it encroaches upon the umbo. The fact that the development of the 

 foramen is thus interrupted before it reaches the circular outline normal to 

 the adult of most paleozoic species, indicates an embryonic character in 

 the adult, and, therefore, a subordinate taxonomic position for the species. 



Plications. These appear only after the first stage of growth is 

 passed and after the first growth-line has been formed. As in Retzia evax, 



