FOSSILS OF THE LOWER SILURIAN. 



69 



Genus SCHIZAMBON, n. g. 



Shell ovate or oblong 1 oval, inequi valve; 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 calcareo-corneous, consisting of a nacreous outer layer with 

 a closely attached inner calcareous layer. Both layers are thought to be 

 punctured by scattered tabulae. Surface smooth or with distinct lines of 

 growth and scattered spines apparently on the outer edges of the laminae 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 tubulse 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 



