114 PALEONTOLOGY 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 layer.'< are thought to be punctured liy scattered tubulae. Surface 

 smooth or with di.stinct 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, 

 he. cit.) 



Type, Schizambon tijpicalis, 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 '""^-ii-*,- 



Siphonotreta, but the surface is minutely spiniterous, aiici- waloott. 



while in »S7/)Ao«o/re/a^5a, Kutorga, which Mr. Walcott is vaive. 



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. 



