52 MEMOIRS OF THE STATE MUSEUM 



tour, there being no such variation in this respect as has been noticed in 

 some other species (e. g. Retzia evax, Meristina nitida) in which appear 

 deviations from the normal, pifoducing a long type and a broad type. The 

 youngest individual detected has a length of 2.25 mm. and a width of 



2 mm., though this may not be regarded as the initial shell on account of 

 the presence of partially developed deltidial plates. From this stage of 

 growth to maturity, the material has afforded every variation in size and 

 structure. 



SPECIFIC CHARACTERS 



Mature Form (plate 6, figs. 13, i-^a, 20, 20a). Atrypa reticularis 



is so widely distributed, historically and geographically, in paleozoic 

 faunas, and is so familiar to paleontologists, that a detailed description 

 here of its mature form is unnecessary. It is sufficient to remark that the 

 prevailing expression at this locality does not precisely conform to the type 

 of A. reticularis, but is more nearly that variety described by Professor 

 Hall (Palaeontology of New York, Vol. 2, p. 271, 1852) under the name 

 Atrypa rugosa. This is evident from the development of the varical lamel- 

 lae, which, over the plications, are infolded into nearly tubular processes, 

 sometimes produced at a strong angle from the shell to a length of a milli- 

 meter or more. On the varices, the plications are covered by fine concentric 

 wrinkles. The average size of mature individuals, 25x25 mm., is less than 

 that usual to the species, when occurring in later, especially Devonian, 

 faunas. 



Incipient Form (plate 6, figs. 12, 12a, 15, 15a). The initial shell, 



or the actual inchoate period in its formation, is not known to us. The in- 

 cipient shell of our series is very small, and can be but a few removes 

 from the initial stage. As just observed, it measures 2.25 mm. in length by 



3 mm. in width, and shows but two concentric stria;, or growth-varices, with 

 a correspondingly slight development of the deltidial plates, so that we are 

 inclined to regard this shell as but two stages advanced from the actual 



