The Rev. Norman Glass — On Athyris IcBviuscula. 497 



posterior border of the spiral. (In the accompanying sketch the 

 primary lamellge and their hook-shaped attachments to the hinge- 

 plate are shown denuded of the rest of the spirals. For similar figs, 

 of my preparations of Whitjieldia tumida vide Davidson, Silurian 

 Supplement, plate vi.) 



The differences between the loop of this species and that of Athyris 

 are thus very strongly marked. 1. Between the simple V-shaped 

 commencement of the loop and the bifurcation there is in Athyris 

 Iceviuscida no roof-shaped expansion and single lamella as in the 

 true Athyris. 2. The short, converging processes which commence 

 the loop in Athyris IcBviuscida at once proceed in an upward direc- 

 tion — whereas in the true Athyris they proceed first in a slightly 

 downward direction before uniting to form the roof-shaped expan- 

 sion. 3. The accessory laraelise at the end of the loop in Athyris 

 leeviiiscula do not extend beyond the posterior border of the spirals, 

 ard are not prolonged down a portion of the dorsal side of the 

 spirals as in the true Athyris. 



I may mention here that there is a median septum supporting the 

 hinge-plate in the dorsal valve of Athyris IcBviuscnla similar to that 

 which is found in Whitjieldia tumida. This median septum extends 

 to a little more than one-thii'd of the length of the shell, and is in 

 some cases visible when the shell is transparent. In addition, how- 

 ever, to this evidence I have clearly proved the existence of the 

 median septum in Athyris laviuscula by rubbing down the umbo of 

 the dorsal valve in a rather large specimen. In this specimen the 

 shell is destitute of spirals and filled with a light earthy matrix in 

 which the darker coloured median septum is very apparent. When 

 describing Meristella l_WhitJiefdia'] tumida in his Sil. Mon. p. 110, 

 Davidson refers to the median septum of its dorsal valve as being 

 visible through the transparent shell, and in his Sil. Suppl. pi. iv. 

 fig. 24, he represents a similar appearance in Athyris Iceviuscida, 

 and yet he makes no reference to the median septum in this species, 

 and does not seem to have been aware of its existence. 



Sowerby's Iceviuscula, according to Davidson, has had a remarkable 

 history so far as the generic positions assigned to it are concerned. 

 After variously figuring in the works of Sowerby, Hall, Bronn, 

 De Verneuil, D'Orbigny, Salter, Lindstroin and Davidson, as Tere- 

 hratula, Atrypa, Rhynchonella, Meristella, Meristina, Spirigera, and. 

 Athyris, I think this species must now after full investigation be 

 finally placed under Davidson's more recent genus of Bifida — vide 

 Davidson, Devonian Supplement, pp. 27, 28. 



Both in Bifida lepida and in Sowerby's Icsviuscula the simple 

 V-shaped commencement of the loop is immediately succeeded by 

 the bifurcation without any intervention of a roof-shaped expansion 

 and single lamella as in Athyris, or of a single lamella as in Kayseria 

 and Whitfieldia. Davidson, in his Sil. Suppl., evidently regarded the 

 spirals and their connexions as the principal if not the sole ground 

 for the determination of the different genera of the spiral-bearing 

 Brachiopoda. Thus, where it was found on investigation that the 

 spirals and loop in several different genera were in all important 



DECADE III. — VOL. Till. — NO. XI. 32 



