148 T. Davidson — Upper Silurian Brachiopoda. 



more level, and the spiral cones more elevated, as we have figured 

 them in p. 9 of this paper. The principle of the variation of shape 

 iu the spirals of certain genera and species of the AtrypidcB, seems 

 to be the providing of siich a form of spirals as should allow the 

 greatest length of coil possible in the interior of the shell ; for 

 example, in Glassia ohovata and G. elongata the ventral valve if 

 anything is slightly more convex than the dorsal, and consequently 

 the spirals are slightly more convex on the ventral side, and the 

 length of the coils on that side is still further increased by the notch 

 or indentation on the ventral slope of the posterior border of the 

 spirals. Again, as we have just seen, in Atrypa marginalis, A. 

 Barrandi, and in the young specimens of A. reticularis, the dorsal 

 valve is neai'ly or quite flat, and this being the case there are several 

 differences between their spirals and those in the full-grown 

 specimens of A. reticidaris, tlae dorsal valve of which is ventricose. 

 First, there are a fewer number of spiral coils, but to allow space 

 even for these some changes were necessary in the arrangement of 

 the spirals, and therefore the principal coils instead of being level 

 are slightly higher on their inner than on their outer sides, whilst, 

 unlike the majority of the full-grown specimens of A. reticularis, the 

 principal coils are some little distance apart, and the ends of the 

 spirals bend over to meet each other. It will be seen at once that 

 these peculiarities in the arrangement of the spirals (the coils being 

 only few in number) make them on their dorsal side to be almost 

 level, and suitable, therefore, to the interior space which they have 

 to fill. 



Through the kindness of Mr. R. P. Whitfield, Mr. Glass and 

 myself have been able to examine the only developed American 

 example of Prof. Hall's Coelospira disparialie, from the collection of 

 the American Museum of Natural History, New York, and we are 

 able to assert that its characters and spirals are so exactly similar to 

 those of Atrypa Barrandi, that we are strongly inclined to consider 

 them as belonging to the same species, and both as referable to 

 Atrypa. I have added these details to show how important it is to 

 study the gradual development of a species from its youngest to its 

 full-grown condition. 



4. Glassia ohovata, Sow., sp. PL V. Figs. 1, 2. 

 Atrypa ohovata. Sow., Sil. sp., pi. viii. fig. 9, 1839. 



We have already described the interior and character of this 

 species. It is variable in shape, the valves being nearly equally 

 and uniformly convex, and almost circular. The front line is 

 straight or slightly curved, surface smooth. Length 5, width 5^, 

 depth 3 lines, 



Glassia ohovata is not an abundant species in Shropshire, but 

 some good examples were obtained by Mr. Maw from the Buildwas 

 Lower Wenlock shales. 



5. Glassia elongata, n. sp. PI. V. Figs. 3, 4. 

 Shell small, elongated oval, valves very gently convex, straight, or 

 slightly rounded iu front, tapering posteriorly, broadest anteriorly, 



