314 T. D. A. COCKKKELL. 



falling under this head are perhaps rather Southern than properly 

 Californian. The Mollusca are very instructive here. There are 

 certain distinct generic types of slugs, Prophysaon and Hemphillia, 

 which belong to the Californian or Pacific province, and these go 

 inland as fiir as Idaho. The Nearctula group of Pupa, which is 

 boreal and Californian, extends southwards into the Colorado Moun- 

 tains, WMth three species. The Vertigo group, on the other hand, is 

 characteristic of the Eastern States, but I never found it living in 

 Colorado. I did, however, find two species of this group, P. ovata 

 Say* and P. gonldli Binn., in a post-tertiary deposit at West Cliff — 

 a fact which I thought might have some significance. From these 

 facts I infer that the fauna of the northern part of the Pacific re- 

 gion is pushing its way inland and southward, and will in the future 

 mingle to some extent with that coming from the southern portion 

 through Arizona. 



The resemblance between the Colorado fauna, and that of the 

 Mississippi basin and further East, always, excepting the boreal ele- 

 ment that comes from the North, is very slight indeed. The great 

 plains to the east of the Rocky Mountains have been as much a 

 barrier as the sea would have been. Many insect pests of the East 

 are only now reaching Colorado, having been taken there no doubt 

 on plants. Sometime ago, in one of the Fort Collins Bulletins, it 

 was noticed that certain Eastern pests occurred in the grounds of the 

 Agricultural College, but not generally throughout the State. The 

 reason of this was, I suppose, simply that they had been taken there 

 on imported plants. 



REASONS FOR NOT ADOPTING THE SONORAN REGION. 



In Dr. Merriam's map the Upper and Lower Sonoran regions in- 

 clude all of North America, except the boreal and transition areas 

 to the North, and the tropical elements to the South. That is to 

 say, the Pacific province is no longer recognized, and the Eastern 

 and Central provinces, so far as they are not boreal, are merged 

 together. It seenis to me that the distribution of the Mollusca, more 

 especially, precludes us from adopting this change. Take the slugs, 

 for instance. Their distribution in the old regions (excluding spe- 

 cies known to be imported) is as follows: 



'■ Yarrow, in the Wheeler Eeport. records P. ovata from Twin Lakes and 

 Saguache, but as he does not mention any of the Nearctula group, which he 

 surely must have found, I suppose it likely that the identification is erroneous. 



