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As mentioned in the immediately preceding account, the small Thomomys umbrinus pullus and the large Cratogeomys were found in the same area.

‘Deinde Pullus Aquilæ nidificabit in summa rupe totius Britanniæ: nec juvenis occidet, nec ad senem vivet.’

In B. musculus, the darker subspecies, B. m. brunneus, B. m. nigrescens, and B. m. pullus, occur in zones of rather constant high relative humidity, whereas the paler subspecies infernatis, musculus, handleyi, and to a less extent grisescens and pallidus, occur in zones of lower relative humidity.

NW San Salvador were considerably darker than paratypes of grisescens and were nearly intermediate in color between nigrescens and pullus.

There is no positive evidence that B. m. grisescens intergrades with B. m. pullus to the south in Nicaragua.