116 PALAEONTOLOGY OF NEW YORK. 



spines, with somewhat thiclver ones scattered among them; in the American 

 specimens the spines, also smooth, are all of about the same size on any given 

 growth-line, increasing somewhat in length from apex to margin. While this 

 difference in the character of the ornamentation of the surface is apparently so 

 considerable we should hesitate to regard the Russian and American forms as 

 identical. At all events a change of name for the American species is required, 

 and it is proposed to designate it Schizamhon? Canadensis, Ami. 



In allocating these Canadian forms and Kutorga's S. fissa, tentatively with 

 the genus Schizambon, it is important not to lose sight of certain apj^arent 

 differences in the structure of the pedicle-passage. In all the specimens of 

 Schizamhon typicalis which we have examined, this passage appears to be a 

 simple oval slit, transecting the shell almost vertically, without forming the 

 short internal tube evident in the Canadian examples. The same character is 

 shown in Mr. Walcott's figures. The shells of S. typicalis are, however, tenu- 

 ous and this divergence may prove to be entirely fortuitous and due to the 

 imperfect retention of the original characters. It is with this reservation in 

 mind that the Canadian specimens are referred to Schizambon. 



The formation of the foraminal groove in all these species is undoubtedly to 

 be regarded as the progressive track of the pedicle-aperture, indicating its suc- 

 cessive positions in different stages of growth. 



The agreement in the external character of the pedicle-passage in Schizam- 

 bon and ScHizoTRETA (Orbiculoidea conica, Dwight, 0. Forbesi, Davidson), is very 

 striking ; indeed specimens of the pedicle-valve of Professor Dwight's species, 

 in which the apex of the shell is distinctly turned toward the anterior margin, 

 have a resemblance to the corresponding valve in Schizambon? fissus. The in- 

 terior extension of the passage in the two genera is different; in the latter 

 (Schizambon), a tube ending abruptly in about the center of the shell ; in the 

 former the passage is not tubular, except where transecting the shell, or when 

 enveloped by internal callosities. (See the discussion of these characters under 

 genus Orbiculoidea.) 



With regard to the internal markings of these fossils, it is difficult to detect 

 features susceptible of a satisfactory interpretation. Specimens of the type- 



