20 ART. 2. — H. YABE : CRETACEOUS 



4) A specimen from tlie Kikumezawa, a tributary of the 

 Ikusliumbets. 



5) A specimen from the Kikumezawa, a tributary of the 

 Ikushumbets. 



The description of t]iis species by Yokoyama is very accurate, 

 but it is based on young specimens and not on adult ones. The 

 full-grown individual of G. tenuiliraturii must have attained a 

 considerable size, for a specimen from a clifiP opposite the mouth 

 of the Panke-moyCibari measures 30 cm. in diameter and is en- 

 tirely septate. By removing the outer volutions, we can trace each 

 stage of development back to the embr3'onal chamber. However, 

 when specimens are very small, it is not always easy to determine 

 whether they represent a young stage of the present species or of 

 other allied forms. That the young and adult specimens belong 

 to one and the same S]3ecies is shown by the abundant occurrence 

 of many transitional forms. For the sake of convenience the 

 writer describes this species in three different stages of develop- 

 ment, although in fact there is no sharp boundary between any 

 two of them. 



The young stage^^ : — Shell discoidal, composed of mau}^ 

 slightly involute whorls. A specimen, 3. 5 cm. in diameter, is 

 composed of six volutions. Whorls transversely reniform, broader 

 than liioh and broadest near the umbilical edo-e. Umbilicus 

 very wide, with a moderately steep wall. 



Surface with very numerous fine striœ and few periodic ribs ; 

 stride unequal in length, more or less S-shaped and acute, much 

 narrower than their interspaces. The longest striae embrace the 

 entire surface of the whorl, while the shortest ones are only on 

 its ventral side ; others are on the ventral as well as on the 



J) Yokoyama: 1. v. PL XV^III. lig. 1;]. 



