Word came out over Bryonet a couple of weeks ago about an online Guide to the North American Bryophyte Genera. The portions that cover liverworts and hornworts have been recently (March 2008) updated. I had not known about this key prior to seeing this announcement. Thus I have not used this key before but here are some of my observations and comments from surfing the website.
First is the Overview Key, which uses parts of the moss plants that can be seen with the naked-eye. I think that this is a really good place to start. After this you are sent off to sub-guides that help you narrow down your identification to genus. There are a couple of different ways to end up at a final answer. You can start by distinguishing pleurocarpous (freely-branching) from acrocarpous (rarely-branching) mosses OR you can look at some larger scale features of either the sporophyte or gametophyte to enter the key. Thus this is a multi-entry key. The utility of this type of key is that you do not know what kind of material a person will have available. If the key only used sporophyte characteristics but your sample does not have any sporophytes then you are pretty much stuck.
They also have some down-sized guides that only cover particular regions of North America. There are three guides for New England +New York, the Midwest, and the West Coast. These guides can be downloaded as word or pdf files, so they could be used in the field. From what I can tell, some of the moss characteristics used in the key can be seen using a hand lens, while there are some for which a microscope is needed.
There are a couple of drawbacks to this guide.
1) There are no descriptions of the genera once you pick one. This is important because it allows you to check the description against your moss sample to be sure you arrived at the best answer.
2) There are no images (photos or line drawings) with this key. This would also be a handy check for your identification. Also images that illustrate important features can be helpful to explain structures that you might be unfamiliar with. Otherwise you have to look up unknown structures in a glossary.
Overall I think that the guide is useful and might be a good place to start for learning the mosses in your area. Enjoy!
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