41 AKT. 2. — H. Y A BE I CRETACEOUS 



fore, Llic outer volution in adult specimens is generally not 

 well ])reserved, wliieli makes exaet measurement very diilieult. 

 These two species are closely allied to each other, and if we 

 compare their dimensions, it is quite evident that there is no 

 niaterial difference between them. The coincidence is not only in 

 dimensions, but also in nearly all other characters if we except 

 the periodic ribs. There is a group of Ammonites in which the 

 shell is smooth until it has attained about 7 cm. in diameter, 

 after which a periodic arrest of growth begins to appear, while 

 another gr(»u]) shows periodic ribs from the very beginning; still 

 it not rarely happens that we can not determine which of these 

 two characters is shown by a specimen not in a good state of 

 preservation. However, in spite of the imperfect nature of the 

 tvpe specimens, we can easily see that Jijjbö's L. .yj/iaeronolufiL 

 is of the first category, while his L. glahruiii is of the second. 

 It is to be added that the writer is, at i)resent, not quite sure 

 whether the absence of periodic I'ibs on the inner volutions is 

 suflicient for specific distinction. Tctragonitc^ cpigoniis Kossmat,^^ 

 a closely related species, is stated to be rather variable in the 

 appearance of the ribs, some young individuals showing them 

 typically, while the larger ones are smooth. Although this 

 statement leads us to assume that the ribs are not of nuich im- 

 portance in the distinction of species, yet the ])resent case must be 

 considered as somewhat different from the above one. As above 

 stated, in T. sphacrouolus the periodic ribs appear after the shell 

 has attained a certain size, while in T. glabrus they are already 

 present when the shell is young. Forms intermediate between 

 these two have not yet been found. Therefore the writer 



1) Kos^i.Vi: 1. 1-. 11. 3'J (i;;5), ri. HI. (XVll), llg. '1,5. 



