Hyatt-] 226 [December 2, 



British Museum, shows, however, that both have similar forms and 

 short living chambers. The septal sutures of Agassiceras Scipionia- 

 num are also strictly Arietian in outline and proportion, and the 

 septa, form and external characteristics of Agass. Icevigatum are so 

 similar to the young of Cor. kridion and roliforme in some varieties 

 that it becomes difficult to separate them. 



Agassiceras laevigatum Hyatt. 



Amm. Icevigatus Sow., Min. Conch., pi. 570, f. 3. 



Amm. Icevigatus Opp., Der Jura., p. 81. 



This species has an exceedingly immature or embryonic form. It 

 seems to complete its growth in five whorls. The mouth has a sim- 

 ple pointed rostrum, the lips slightly flaring with a broad shallow 

 constriction between the edge and the main body of the shell, very 

 similar to Oppel's type of Amm. planorlis, which, by the bye, is 

 not correctly figured by him. 1 



Variety a is smooth during four whorls at least and flatter and 

 thinner than the other varieties, and the umbilicus is therefore not so 

 deep. These pass by insensible gradations into the next. 



Variety o is smooth only during the first three or three and a half 

 whorls, and then the sides are broken by a series of immature folds 

 like those of variety plicatum of Psiloceras planorbe. The younger 

 whorls are generally wider than in either of the other varieties and 

 .the umbilicus therefore deeper. These fade insensibly into those of 

 the next variety. 



Variety c has the pilae much more distinct but the period at which 

 they are developed is the same as in the preceding variety. An- 

 other peculiarity of this variety is the tendency of the pilse to cross 

 the abdomen, forming slightly prominent ridges. This, like all other 

 characteristics, is found to a greater or less degree in the other 

 varieties. 



Variety d is founded on the presence of a faintly defined siphonal 

 line. This includes members of the other varieties, regardless of the 

 time at which the pilse are developed, their greater or less promi- 

 nence, and the breadth or thinness of the whorls. This variety also 

 fades off into all the rest in respect to its distinctive peculiarity, the 

 difference being wholly one of degree. 



The young have at first the abrupt umbilical edges, common also to 

 the young of other species; these give considerable depth to the 



i Oppel's figure gives the lateral curves in an exaggerated form, and the in- 

 dented collar too deep. 



