B737 on visual approach to LAS fails to remain above Class B floor because of early descent.

Narrative

We were given the visual approach to Runway 25R on base about 25 DME LAS. At this time; the Captain and I discussed the issue of staying above the Class B floor on the approach and he had his approach plate out as well as the Class B chart for LAS. I had only the ILS Runway 25R approach plate out. As we proceeded on the visual; he would give me the DME and altitude I could descend to in order to stay in the Class B. At one point I asked if the altitude and DME were correct for a descent to 4;500 FT and he said it was. I continued the descent because he was looking at the Class B chart. Mine was not out; and my main concern was a stabilized approach. Approaching 4;500 FT; the Captain realized that; in fact; we had descended early and were below the Class B momentarily between 15-20 DME. We leveled; confessed our mistake to the Tower; re-entered the Class B; and continued a normal approach and landing on Runway 25R. We were aware of the Class B floor issue on the visual approach. However; in hindsight; as the pilot flying; I did not brief this issue on my approach brief and had poor situational awareness of where the floor was. I failed to back up the Captain by not having my Class B chart out which allowed an honest chart misread mistake to not be caught. I will become familiar with the Class B airspace in LAS and will have the Class B chart out or mark on the approach plate where the Class B limits are. I will also brief the Class B floor issue on approach briefings from now on.