136. Scientific Proceedings, Royal Dublin Society. 
from opposite sides of the ribs meet,! and are cemented with the 
ridges into a common calcareous plate, supported by the lines of 
tubercles. When stripped of this crust, the keel appears to be a 
beautiful chain line of tubercles, with a line of pores on either 
side, as in other species of Fenestella.”’ The fenestrules are ‘‘ ob- 
long, oval, frequently closed at bottom, or opening by a slight slit 
on the reverse surface.” I find that Prout’s measures give about 
20 fenestrules to 1 centimetre, measured longitudinally, and 24 
when measured transversely. The cells (zocecia) are “large, about 
two to each fenestrule.”” The “cells” of the outer sheath are, how- 
ever, said to be “invisible to the naked eye.”. This is probably 
due to the excellent condition of the specimen, the external opening 
in the outer sheath being smaller than the internal in the examples » 
of Hemitrypa that I have examined, the structure being, in fact, a 
minute cup perforated at the bottom or outer eud, and liable, there- 
fore, to yield a larger external opening when worn down from the 
outside (see Pl. vui1., fig. 3). I was led, indeed, for a short time to 
suppose that these openings in the ‘sheath might have been, in Irish 
examples, completely closed over on the outer side. 
Prout was the first, I believe, to show that the outer layer was 
-supported away from the inner one by processes from the carinz 
of the Fenestellid ribs; yet evidence could have been obtained 
by grinding down the end of any reasonable Irish specimen and 
examining the resulting section with a pocket-lens. 
The reasons that prevented Prout from referring his species to 
Hemitrypa were founded, it seems to me, upon a misapprehension 
of Phillips’s original description. 
The importance of Hemitrypa was, however, never overlooked 
in the United States. Hall has described the species prima, dubia, 
Nerma, and biserialis,’ all from Silurian (Niagara and Lower Hel- 
‘This is obscurely worded, but appears to mean that the “ cells’? of the outer 
sheath, borne by the tubercles of adjacent ribs, meet above the middle line between 
those ribs. 
* H. prima was described in the Twenty-sixth Annual Report of the New York 
State Museum of Natural History, p. 98. H. dubia is figured in the Twenty-eighth 
Report (1875), pl. 11, figs. 17-21; but in the “‘ State Museum Edition ’’ of the Twenty- 
eighth Report (1879), p. 123, it is described as Fenestella ambigua. Fenestella (Hemi- 
trypa) Nervia is described in the Thirty-second Report (1879), p. 173, and F. (Hemi- 
trypa) biserialis in the same work, p. 174. 
