Answer: Diphyllobothrium species (broad fish tapeworm)
The most common species found in the United States is Diphyllobothrium latum, although as Anon points out, there are other species of Diphyllobothrium (e.g. D. pacificum, D. ursi) that can also infect humans and they are essentially indistinguishable by morphologically alone. Therefore, it is best to simply identify these as "Diphyllobothrium sp."
This case is very characteristic of what we normally receive in the lab; that is, specimens that are not well-preserved and sometimes wrapped up in toilet paper or other materials. However, the characteristic features are still identifiable: proglottids that are wider than they are long with a central darkly-staining uterus and ovaries. As Florida Fan points out, the expressed eggs are also characteristic for this genus, with a clearly recognizable knobby protrusion (abopercular knob) and a less easily seen operculum.