1870.] OO [Hyatt. 



This unsymmetrical form is less common than in D. confusum, at 

 least in the collection I have examined; and I have never found 

 such specimens as are mentioned by Quenstedt, which, though 

 unsymmetrical in the young, become symmetrical in course of growth. 

 Most of the specimens that I have yet seen have this tendency to form 

 a spiral, expressed on or towards the right side, — remembering that 

 the external periphery is the abdomen and no t t the dorsum as is gen- 

 erally supposed — on the same side, in fact, as the want of symmetry 

 so frequent in the lobes of Psil. psilonotum. 



Though this species has been placed in the same genus as M. 

 confusum, I think it can only be considered as one of a different series 

 of planicostan forms, those with two lines of lateral tubercles. 



Variety mixtum. 



The two specimens of this so called species, if the label from 

 the Museum of Stuttgard is reliable, agree very closely with the fig- 

 ure of Amm. polymorphus quoted above, and in their septa with 

 M. biferum of the same age, as well as with some of the other figures 

 of Amm. Polymorphus given by Quenstedt. It may be a variety of 

 that species. One specimen has the Turrillite deformity so often 

 found in M. biferum. 



Microceras laticosta. 



Microceras laticosta Sow.,Min. Conch., vol. vi,p. 106, pi. 556, fig. 1. 

 " brevispina Sow., " " " " fig. 2. 



" sinuosum Hyatt, Bull. Mus. Comp. Zoology, no. 5, p. 82. 



. " maculatum Hyatt, Op. cit., p. 82. 



The young of this species resembles Microceras biferum so closely 

 in all its characteristics, that it does not differ so much from it, as the 

 different varieties of that species do among themselves. The form 

 of the whorl in most individuals begins very soon to exhibit a flatness 

 of the abdomen and sides and a sharp bending forward of the pilae 

 on the abdomen, which are the only distinctive characteristics. The 

 septa are not precisely similar. The differences, however, increase 

 with age as the septa become more complicated and the pilse more 

 prominent. Two rows of tubercles are acquired in some specimens 

 during the adult stage. The abdomen is still deeply sinuous as in 

 the typical M. biferum. 



There are two forms of this species, one flatter and less robust than 

 the other, which I have called M. maculatum. This has no spines, 

 at least none are apparent upon the casts. 



In variety sinuosum, the age at which the tubercles are assumed 



