BRACHIOPODA. 153 



observed in Atrypina, though the introversion of the spirals is less complete 

 in the latter. Glassia stands in the same relation to Atrypina as Protozyga 



Fig. 145. 

 lirachidiuni of Glassia oborata, Sowerby. (Davidson.) 



to Hallina ; Glassia and Protozyga having essentially smooth exteriors and 

 the others a plicated surface. Other differences will probably be found in 

 Glassia and Atrypina, and these are indicated by the peculiar structure of the 

 deltidial plates in the former, as described above. Davidson recognizes three 

 species of Glassia, G. obovata, Sowerby, G. elongata, Davidson, from the Wen- 

 lock shales, and G. Whidbornii, Davidson, from the middle Devonian of Torquay. 

 The continuation of this type of structure into the Devonian is of interest as 

 being the only instance of the passage of the primitive zygospiroid structure 

 beyond the limits of the Silurian. Glassia is represented in the lower Silurian 

 of North America by an undescribed species found by Dr. C. Rominger in a 

 drifted boulder of Trenton limestone, near Ann Arbor, Michigan, in association 

 with Cyclospira bisulcata. This species, Glassia Romingeri, has the smooth, con- 

 vex valves and the introverted spirals of G. obovata, but is a more elongate 

 shell. (See Supplement for description.) 



