116 DEVONIAN FAUNA. 
of the dorsal valve; ribs rather coarse and frequently divaricating close to the 
margin ; shape transversely oval, and generally flattish even in large specimens. 
3. A. trigonella: beak large, erect, narrow; area large, narrow; deltidium 
large; foramen large, oval, very distant from the apex of the dorsal valve; 
size small; shape subcircular, flattish; ribs few, very steep and subangular, 
divaricating, and becoming very twisted and irregular near the margins; surface 
covered by distant and regular foliaceous lines of growth. 
4. A. aspera: beak incurved, small, sometimes showing foramen; shape 
circular, not transverse, not very globose; ribs very few and coarse, broad, very 
much rounded, and never divaricating; crossed by few regular, simple, nodulous 
ridges of growth, surmounted by foliaceous lines. 
a. “4.” seems distinguished from ‘‘3” by its simple, lower, and rounded 
rays, and its smaller beak. The coarseness of its rays distinguishes it from 
Seales same ct 27 
B. 3”? seems distinguished from “1” and ‘*2”’’ by its constantly smaller 
size, and its much coarser and steeper ribs. Its large beak and area separate it 
trom =.” 
y. Young specimens of “1” and ‘*2” differ widely from each other in the 
character of the beak and area, and it is only in larger specimens that there is 
room for doubt. In the case of some full-grown specimens of “2” (agreeing 
with Sowerby’s var. compressa and Sp. ? affinis, Sow.) the differences are as 
marked as in the young shells, as they are flat and often transverse, and the area 
is large and flat; in others, however, the shell is more globose and the beak 
incurved (cf. Davidson’s Mon., vol. iii, pt. 6, pl. xi, fig. 1), and as in large speci- 
mens the beak is frequently destroyed or hidden and the surface often defective, 
they are sometimes most difficult to allocate. 
Returning now to A. reticularis proper, we observe that it is (including 
doubtful specimens) extremely common, and often reaches a very large size. 
There are about 100 specimens in my Collection from Lummaton, the largest 
of which measures 50 mm. long, 70 mm. wide, 33 mm. deep; this, however, is 
one of the doubtful examples. A typical specimen is 34 mm. long, 37 mm. wide, 
and 22 mm. deep. There are seven specimens in the Woodwardian Museum. 
From Wolborough there are three specimens in Mr. Vicary’s Collection, seven in 
the Museum of Practical Geology, and seven in the British Museum. 
The ribs vary considerably in coarseness and number, but are typically very 
fine. I have about seven specimens which show the expanded frills. In two 
cases these reach a width of 26 mm. from the circumference, and even then are 
ended only by the fracture of the stone. There are several distinct layers of 
fringes, but they appear only to have been developed near the margins of the 
valves. 
