114 
PALjEONTOLOGY OF NEW YORK. 
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 tubulae. 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 depres¬ 
sion extends obliquely outward. No other markings were observed. In the 
interior of the dorsal valve a pair of anterior central muscular scars terminate 
their path of advance from the beak, a slight rounded ridge rising on the 
central line; posterior 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.” (Walcott, 
loc. cit .) 
Type, Schizambon typicalis, Walcott. 
Observations. This peculiar genus presents an additional phase in the 
variation in position of the pedicle-aperture. We have seen, in Kutorgina, 
Acrothele, Acrotreta, etc., the stages of transition from the marginal pedicle- 
slit of the obolelloids to the apical aperture in Siphonotreta, a group in which 
the subapical area loses its specilization and the progressive pedicle-furrow or 
ridge. Schizambon has the foramen in front of the umbo, 
and by the advancing growth of the shell, the opening 
always remaining at the same relative distance from the 
margin, an external groove, tapering toward the beak, is 
left behind it. The affinity of this genus with Siphono¬ 
treta is very close. In the type-species, the outline and 
contour is slightly different from that usually met with in 
Schizambon typicalis. 
Siphonotreta, but the surface is minutely spiniferous, After walcott. 
. Fig. 65. Exterior of pedicle- 
while in Siphonotreta fissa, Kutorga, which Mr. Walcott is valve, 
disposed to regard as congeneric with Schizambon typicalis , the similarity with 
typical forms of Siphonotreta, in all external respects save the character of 
the pedicle-channel, is complete. 
