What species is this turtle in Houston?
That's an Alligator snapping turtle, Macrochelys temminckii.

Here is another picture of it (from the same Wikipedia page linked above):



It should not be mistaken for the Common snapping turtle, which belongs to a different Genus.

There is a problem, though: in 2014 Thomas et al. divided the Genus Macrochelys (which originally contained only one species, Macrochelys temminckii) in three species: Macrochelys temminckii itself, Macrochelys suwanniensis and Macrochelys apalachicolae. 

Therefore, narrowing this down to the exact species (according to the current taxonomy of the Genus) may be way more complicated.



Source: 


Thomas, T, Granatosky, M, Bourque, J, Krysko, K, Moler, P, Gamble, T, Suarez, Leone, E, Enge, K, &amp; Roman, J, 2014. Taxonomic assessment of Alligator Snapping Turtles (Chelydridae: Macrochelys), with the description of two new species from the southeastern United States. Zootaxa 3786 (2): 141–165