Thomas Davidson — Spiral-Bearing Brachiopoda . 



9 



Fig. 8. 



more fully determined. Nothing further than that Itetda Salteri was 

 provided with spiral appendages had been discovered before Mr, 

 Glass, at my request, worked out their mode of attachment to the 

 hinge-plate and their connecting lamellae. After many experiments 

 Mr. Glass was able to show in the clearest possible manner that 

 the spirals were attached to the hinge-plate and connected with 

 each other in a similar manner to Nucleospira pisum. The only 

 difference seems to lie in the spirals themselves, namely, that in N. 

 pisum they are formed of five or six convolutions only, while about 

 ten can be counted in each spiral in Retzia Salteri. 



Family Atrypid^, Dall, 1877. 



Genus Atrypa, Dalraan, 1828, type A. reticularis, Linne. 



The interior characters of this genus have been completely de- 

 scribed by Mr. K. P. Whitefield 

 and Prof. J. Hall. Its spiral 

 appendages, first attached to the 

 hinge-plate, are placed side by 

 side, with their extremities 

 facing the middle of the bottom 

 of the dorsal valve, as may be 

 seen in the accompany ing Figure. 

 Mr. Glass has also developed 

 the complete interiors of Atrypa 

 reticularis from English speci- 

 mens, and his worked specimens 

 will be fully described and 

 illustrated in my Silurian Sup- 

 plement, The principal stems 

 of the spiral coils at a short 

 distance from their attachment 

 to the hinge-i^late are connected 

 by a narrow band, the branches 

 from either side converging 

 downwards into a V-shape, and 

 each branch being slightly 

 turned up at its extremity. This band seems continuous in many 

 specimens that have been operated upon, but, as stated by Mr. W, 

 Gurley, in some individuals this loop or band would appear to be 

 disunited, the inner ends opposing each other without being actually 

 connected, I must also observe that the admirably prepared 

 specimens by Mr, Glass show that the spiral appendages in the 

 Dudley specimens of A. reticularis commence by forming A'ery large 

 coils, but that these soon become much smaller in diameter, form- 

 ing two comparatively narrow cones, and that the extremities of these 

 cones are closer together in some specimens than in others, and that 

 sometimes one spiral cone seems a little longer than the other, one 

 cone, for example, showing only fourteen convolutions, whilst its 

 companion cone has as many as sixteen. 



Atrypa reticularis, from an American 

 specimen lent to me by Prof. J. Hall. 



