Hyatt.] 40 [October 5, 



essential point, — the presence of median minor cells which equally 

 divide the superior lateral lobes. This characteristic, though it may- 

 be absent in many specimens, is so constant that it prevents the 

 direct connection of the young of this species with the young of 

 C. torus or 0. raricostatum, which it otherwise so closely imitates. 



The resemblance to the young of Microderoceras Birchii is not so 

 close, however, in external features, though the septa are very closely 

 allied. The Turrillite variety is quite common in this species, 

 whereas it is not so common in the true Amm. planicosta Sow. 



One of Quenstedt's figures of this species represents a young shell 

 decidedly keeled. This, I think, cannot be of the same species, and 

 his accompanying descriptions do not justify its associations with his 

 Amm. Bronnii. 



The variations in the lobes are excessive. The median cells of the 

 superior lateral lobes are usually largely developed, often, though not 

 invariably, retaining the youthful or one-sided aspect which they 

 have in the young of Deroceras planicosta and M. Birchii. In some 

 specimens, however, they are very small, and the lobes are unequally 

 divided by two very minute minor cells. These lobes, in other words, 

 may be equally divided, or have all the gradations from this to a state 

 of unequal division. The same lobes are either longer, equal to, or 

 shorter than the abdominal lobe, but seem invariably to greatly ex- 

 ceed the inferior lateral lobes. 



Deroceras densinodum. 



Amm. armatus densinodus Quenstedt, der Jura, p. 105, pi. 13, 

 figs. 9, 10. 



D. densinodum Hyatt, Bull. Mus. Comp. Zool. , no. 5, p. 84. 



This species does not apparently occur in the middle Lias as stated 

 in my paper in the Bulletin of the Museum of Comparative Zoology, 

 but only in the lower Lias. The mistake resulted from an erroneous 

 reading of the label on the specimen. It may be only a variety of 

 D. confusum, but the young differ somewhat, the abdomen is nar- 

 rower and the septa are invisible in the single specimen of D. densi- 

 nodum in the possession of the Museum. 



According to Quenstedt's figure this species is really an armatus in 

 which the young is pilated or ribbed at a very early period, instead 

 of being smooth as in armatus proper. Having only the young it is 

 impossible to say much about the affinities of the shell ; it is, how- 

 ever, evidently a member of the planicostan group or series of 

 Deroceras. 



