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. 
Brachidium of Glassia dbovata, 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. ohovata, 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.) 
