78 
Horizon and Localities . — With the exception of one from Kemps ej 
all the individuals of this species have been found in the Yass District, 
on the hanks of the MuiTumbidgee. The majority occur in a dark grey 
compact limestone ; one is in a rather hard yellowish sandstone, of Avhich the 
surface, on being oxidised by the air becomes covered with a thin layer of 
oxide of iron whi(di colours it red. I am led to believe that all these belong 
to the Devonian. 
2. Ateypa desquamata, Sowerhij. 
jLtrypa descpianiata , Sowerby, 1840, Trans. Geol. Soc. London, V (2), pi. 66, figs. 19, 20. 
,, ,, var. campana. Idem, 1840, ihid., pi. 56, figs. 21, 22. 
Spirigerina desquamata, McCoy, 1852, Brit. Pal. Foss., p. 378. 
jdtryqja ,, T. Davidson, 1865, Mon. Brit. Dev. Bracb., p. 58, pi. 10, figs. 9-13, 
and pi. 11, figs. 1-9. 
The characters of this Atrypa are so similar to those of Atrypa 
reticularis, that it is often very hard to distinguish the shells belonging to the 
one from those referred to the other. My learned friend, Mr. Davidson, also 
expressed his doubts that A. desquamata is anything hut a variety of A. 
reticularis. He observes that the chief differences between the two forms 
consists ill that the first has a well-marked area and very noticeable foramen, 
while this foramen is usually hidden in the second by the strong curvature 
of the beak ; it is otherwise less globose, and the growth lamellcC are smaller 
and less well marked. But these are not in this case very salient characters 
that may not he easily modified in accordance ivitli the conditions under 
which the animal has lived. 
3. Atrypa plicatella, L. G. de Koninch. 
PI. III., fig. 4. 
Shell oval in shape, elongate, rather small. Dorsal valve fairly regu- 
larly arched, without any apparent fold ; surface ornamented with eleven or 
thirteen radiating ribs, rounded, separated from one another by furrows of 
the same width as the folds, of which the median is a little more j)rominent 
than the others and plays the part of a fold. Ventral valve a little less 
convex than the opposite valve, slightly sinuated in the median j)ortion ; 
