Cactus Identification and Care Question
I bought this cactus last month and was hoping somebody would be able to help me identify the specific type. 

Now for the care piece -the ridges towards the top of the cactus in the attached image seems to be flimsy. I've watered once since purchase on Sept 13 (Sept 24), so I would assume over watering is not the issue. Should I be worried about the flimsy top (is that the same as "soft" which usually signals root rot?). And if so, are there ways to check for health besides taking it out of the pot because that's not too easy. Other notes - the soil is completely dry at the moment down at least two inches. I’ve also just picked up a grow light on the chance that insufficient sun is the culprit.
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    Greg Additional picture to help with identification
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