14 ART. 2. — H. YABE : CRETACEOUS 



it is not usually visible unless the specimen is in a very favorable 

 state of preservation. 



All the species of Gaudryceras seem to be very imperfectly 

 known. This is partly due to the fact that although they are 

 very common especially in the upper Cretaceous deposits of the 

 Indo- Pacific region, the specimens usually belong to immature ani- 

 mals, and consequently closely resemble one another so that 

 the determination is not only very difficult, but often quite im- 

 possible. Äloreover, the aspect of the shell of this genus is so 

 different in its younger and older stages, that without a large 

 series of specimens for conq^arison, the larger and smaller forms 

 are often liable to be separated into distinct species. There are 

 already several instances of confusion among the species of this 

 geiius. The full specific character, as it seems to the writer, 

 appears always in the mature stage or at least after the shell 

 has attained the middle stage of its life. The differences existing 

 among the young indviduals of different species are very slight and 

 often apparently less marked than those between individuals in 

 one and the same species, while the shell of the full grown forms ol 

 each species is w^ell characterized and easily distinguishable from 

 its allies. However, owâng to the rapid enlargement of whorls in 

 adult specimens and the extreme thinness of the shell, it is not 

 always easy to get specimens with a well preserved shell. 



The largest Japanese species is Gaudryceras tenuiliratum, 

 one of the specimens measuring over 30 cm. in diameter and 

 being entirely septate, but still showing the sharp ribs as usual. 

 The following is what Fr. Schmidt^^ wrote about an Ammonite 

 from the Cretaceous of Saghalien which he identified with G. Sacya. 



1) Fk. Schmidt: Die Petrefakten der Kreklefonnation von der Insel Suehalin. 1S73. 

 P. IG. 



