322 PROCEEDINGS OF TEE NATIONAL MUSEUM. vol.45. 



The Atrypa didyma' described by Sowerby is smaller than the spe- 

 cies as afterwards identified by Davidson. The dimensions given by 

 Sowerby are: length, 5 lines; width the same, whereas Davidson gives 

 the dimensions of the two specimens cited as 11 by 10 and 10 by 10 

 lines. The dimensions of two specimens of Whitfleldella edmundsi are 

 5i by 6 and 6 by 5^ lines, or approximately the size of Sowerby's speci- 

 mens from the Aymestry. Our species approaches in form more 

 nearly that of Davidson's specimen, figures 6 and 7, from the Wenlock 

 than the Aymestry form figured by Sowerby. The species as figured 

 by Hall and Clarke and named Whitjieldella didyma Dalman ap- 

 proaches more nearly Davidson's figure 4, also a Wenlock form, but 

 the figure does not show a furrow in the brachial valve. Sowerby 

 describes his species as having a furrow in each valve. Davidson 

 includes in his definition those with furrow in each valve and those 

 with "valves regularly and' moderately globose." 



In form and size (external) TF. edmuTidsi resembles most closely 

 Atrypa nucleolata Hall, which is listed by Schuchert as Whitjieldella ( ?) 

 nucleolata. In Whitjieldella, however, there is a distinct median sep- 

 tum in the brachial valve supporting a concave hinge plate. There 

 may be a simple pointed process terminating the loop as in that 

 genus but the termination is missing in all our specimens. The 

 probability that it is simple is inferred from the cast of a single per- 

 foration in center between the inner ribbons of the spiral coil. 



Atrypa nucleolata was regarded by Hall as very closely approach- 

 ing A. nitida and distinguishing characters mentioned are its less 

 elongate form, and the more conspicuous furrow in the dorsal valve 

 of the Coralline species, and in the description furrows on both 

 valves are recognized, though not always present. These resem- 

 blances are of external characters, and the close relationship existing 

 between A. nitida Hall and A. didyma Dalman was recognized by 

 both Davidson and HaU and Clarke, the difficulties arising in deter- 

 mining the internal characters. I have no evidence by which to 

 distinguish the internal characters under dispute. One of our speci- 

 mens exhibits the spiral brachial supports and the loop running inte- 

 riorly connecting the two inner arms but the exact nature of their 

 termination is not evident. The species therefore comes within the 

 definition of M. didyma Dalman in its external and so much of 

 internal character as can be made out. It may be identical with A, 

 nucleolata HaU, but if so that name is probably a synonym for M. 

 didyma (Dalman). Generically, I am inclined to beheve, it should 

 remain in the same genus with Meristina tumida. In one specimen 

 I find a broad jugum and accessory plates parallel to the first volu- 

 tion of the brachia, like Athyris. The characters require further 

 study. 



