FOSSILS OF THE LOWER SILURIAN. 69 
Genus SCHIZAMBON, n. g. 
Shell ovate or oblong oval, inequivalve; valves inarticulate; larger or 
ventral valve most convex, with a short obtuse beak at the cardinal margin. 
Foramen oblong and opening on the summit of the valve, no area nor delti- 
dium; cardinal edge thin; smaller or dorsal valve nearly as: convex as the 
larger, slightly flattened along the median line. 
Structure caleareo-corneous, consisting of a nacreous outer layer with 
a closely attached inner calcareous layer. Both layers are thought to be 
punctured by scattered tubule. Surface smooth or with distinct lines of 
growth and scattered spines apparently on the outer edges of the laminz or 
lines of growth. 
The interior of the larger valve shows the oblong foramen in a slight 
elongate depression and a pair of muscular scars just in front of it on each 
side of a slight longitudinal depression; from near the beak on each side of 
the foramen, a shallow sharply defined depression extends obliquely out- 
ward. No other markings were observed. In the interior of the dorsal 
valve a pair of anterior central muscular scars terminate their path of ad- 
vance from the beak, a slight rounded ridge rising on the central line; pos. 
terior to these a larger pair occur, and still beyond and more posterior a 
third pair, a narrow rounded ridge extending obliquely down from the beak 
on each side between the central and lateral scars. 
The above description is based on a large number of specimens. Nu- 
merous interiors of the valves occur owing to the rough outer surface cling- 
ing to the fine arenaceous limestone matrix. In the interiors little pits 
occur scattered irregularly over the surface, and on the outside similar pits 
are seen. It is difficult to determine whether these communicate with each 
other. Judging from their appearance and also from a cross-section of the 
shell that shows minute tubule nearly through the shell, we are inclined to 
consider the shell as punctate. The little, short spines of the outer surface 
have the form of those on Siphonotreta; whether they are hollow or not has 
not been determined. 
The most nearly related genera are Acrothele and Acrotreta. It is 
unlike Acrothele in the structure of its shell, in the position of the opening 
