The Isopod that I have to describe is remarkable from the fact that it has only six pairs of legs, whilst the normal number is seven pairs. In many Isopoda the young have at first only six pairs of legs, the last thoracic segment being but slightly developed and destitute of appendages,^* and hence it might, at first sight, be thought that the animal I have is only an immature form. This, however, I think can hardly be the case, for there is nothing embryonic about the appearance of the animal, and moreover, I have examined altogether twenty-two specimens, varying in length from .16 of an inch to .46 of an inch, and these all agree in wanting the last pair of thoracic legs. These specimens were obtained at various times from January up to October, 1881, and I think it is hardly possible that these can all be immature forms, and that during the whole time not one mature form should have been obtained. If it is, therefore, a mature form, the absence of the last pair of thoracic legs must I suppose be due to arrested development.