Hyatt.] 38 [October 5, 



parted with, is subject to considerable variation, sometimes an entire 

 whorl earlier or later. 



Deroceras ziphius. 



Amm. ziphius Ziet. Verst. Wurt., p. 6, pi. 5, fig. 2. 



D. ziphius Hyatt, Bull. Mus. Comp. Zool., no. 5, p. 81. 



This species occupies a position precisely intermediate between 

 D. confusum, D. Dudressieri and the true armatoid, large, single- 

 spined species like the typical armatus. It has, in the young, an 

 abdomen similar to the planicostan abdomen observed in the two first 

 named species, and in the adult it drops this characteristic for an 

 abdomen similar to that found in D. Dudressieri, assuming at the 

 same time a row of large single tubercles. My observations were 

 made upon a single specimen, but they are confirmed by Quenstedt 

 who takes a similar view of the relations of this species from more 

 extended experience. 



Deroceras planicosta Hyatt. 



Amm. planicosta Sow., Min. Conch., vol. I, p. 167, pi. 73. 



Microceras planicosta Hyatt, Bull. Mus. Comp. Zoology, no. 5, 

 p. 80. 



For the first four whorls this species is smooth, and the whorl is 

 remarkably broad, with gibbous sides. On the fifth whorl the pilse 

 are introduced as depressed folds, and gradually increase in size. 

 Spines are never developed in the majority of the specimens, but in 

 a few cases they may be noticed rising either on the cast or the shell, 

 during the third quarter of the sixth volution and becoming quite 

 prominent on the last quarter. The number of pilse on a single 

 whorl, the time at which they cross the abdomen, and the pres- 

 ence or absence of tubercles, vary remarkably. The abdomens of 

 some specimens may be crenulated by the first pilae, or they may re- 

 main smooth even throughout the fifth whorl, and the number of 

 pilse vary from twenty in some to twenty-six in others. 



The septa also in the young, instead of retaining the usual propor- 

 tions of the superior lateral cells and lobes, almost obliterate these 

 two which are represented, as in the adult of Coroniceras tenue, by 

 a row of minor lobes and cells. It, however, still retains the peculiar 

 median cells of the superior lateral lobes, which are so characteristic 

 of the three series of planicostan forms. These begin to show them- 

 selves as lateral expansions or crenulations of the superior lateral cells 

 on the latter part of the fourth or early part of the fifth whorls. The 

 subsequent division of the superior lateral cell into two unequal por- 



