124 



THE NAUTILUS. 



laid great stress on such differences in his study of this group, rely- 

 ing on (he well-recognized principal that " marked differences in the 

 embryos and young of a class of beings are specific." The young 

 milesii when ready for extrusion is uniformly larger than the young 

 of decisa {ynilesii 4.75x3.50, dccisa 4.25x3.50 mill., specimens fig- 

 ured). The shell is more slender and noticably more elongated, the 

 apex being well elevated above the next whorl, while in decisa the 

 apex is depressed, giving a planorboid shape to tlie apex, and rises 

 scarcely if at all above the second whorl. This difference, shown by 

 figs. 5 and 6 from the two species as found together in the Detroit 

 River, is characteristic and persistent. There is no substantial vari- 

 ation in the young of the Detroit River milesii, and none in fourteen 

 different lots of decisa, from localities as widely separated as Port 

 Cram, N. J., Detroit River, Grand Rapids and Marquette county, 

 Michigan. The color in both forms is the same, a pale green, and 

 both have raised revolving lines of epidermal tissue. 



It is unfortunate that no detailed anatomical examination of either 

 species has ever been published. It is quite possible that when that 

 is done, other differences will be found which will confirm the view 

 herein expressed. In the meantime, it certainly seems advisable to 

 recognize the specific validity of this interesting form. 



EXPLANATION OF PLATE V. 



1. Campeloma milesii Lea. Detroit River, Michigan. 



2. Campeloma milesii Lea. Detroit River, Michigan. 



3. Campeloma milesii Lea. Detroit River, Michigan. 



4. Campeloma milesii Lea. Saginaw Bay, Michigan. 



5. Campeloma milesii Lea. (Young.) Detroit River, Michigan- 



6. Campeloma decisa Say. (Young.) Detroit River, Michigan. 



7. Campeloma milesii Lea. Charlevoix, Michigan. 



8. Campeloma milesii Lea. Carp L., Emmett Co., Michigan. 



9. Campeloma milesii Lea. Pine River, IMarquette Co., Mich. 



10. Campeloma exilis Anth. Illinois River, Illinois. 



11. Campeloma subsolida Anth. Clinton River, Macomb Co., Mich. 



12. Campeloma decisa Say. Pine River, Marquette Co., Mich. 



