Anounced September 2015

Rewrite and update April 2018: This is a Tomahawk with 128 SerDes running at 20 Gb/s. 64 ports not withstanding, this is not a Tomahawk2. The 7260QX-64 is covered in a combined data sheet. A surprise is that this the only QSFP switch that cannot use a splitter to farm ports out to 4 x 10 Gb/s. It appears the SerDes are locked at 20 Gb/s so that a pair of lanes has the right speed to make a 40 Gb/s port along with an undisclosed gearbox.

An article September 14, 2015 in nextplatform.com Timothy Morgan said:

There is a variant of this box called the 7260QX-64 that uses the Tomahawk ASIC but only supports 40 Gb/sec speeds (using QSFP+ ports), only 5.12 Gb/sec of switching bandwidth, and only 16 GB of buffer capacity. The aim of this machine is to be a lower cost spine switch for 40 Gb/sec networks that also burns less power than the older 40 Gb/sec switches based on 10 Gb/sec lanes and the Trident-II chips. This switch will cost around $1,000 per 40 Gb/sec port, says Hull, at list price. Port-to-port latency is at 550 nanoseconds. The 7260QX-64 will be available in the fourth quarter. He said 16 GB but he meant 16 MB. The variant to which Tim is referring references the 7260CX-64 which Tim says contains 4 Tomahawks. I think he has not accounted for the ASIC interconnect fabric.

Here's an Arista family family overview for Tomahawk.