The first variety corresponds with the generic character, and the third approaches the genus _Porites_; yet so unequivocally identical are they, that I have seen them all united in the same mass, and perforated throughout by the osculi. The identity is further obvious by the perfect gradation which renders them inseparable. With respect to the transverse septa, I think their presence may be accounted for by supposing that as the animal elongates its tube in consequence of an increase of growth, or in order to maintain an equal elevation with the adjacent tubes, (rendered necessary by the origin of young tubes in the interstices) it gradually vacates the basal portions of its tube, and sustains itself at the different elevations, by successively uniting the parietal lamellæ so as to exclude the vacuity. That this is probable, we may infer from a similar procedure on the part of several species of testaceous mollusca. Thus some Linnæan _Serpula_ become camerated, and a familiar instance presents itself in the _Triton tritonis_, the animal of which adds successive partitions to the interior of the spire, as that part becomes too strait for the increasing volume of its body. If the above supposition proves correct, the organs of communication which pass through the osculi, can hardly be in common, but must rather connect the animals by simple contact only, otherwise these parts would be broken when the animal changes its place by vacating the inferior part of the tube. The third variety is then the state of that portion of the tube which is inhabited by the body of the animal, and not yet interrupted by the septæ. From the above observations, it is evident that this species, and probably the entire genus _Favosite_ under which I have placed it, will not arrange properly with the _Tubipores_, _Millepores_, &c. but must be transferred to the _Polypiers Lamellifères_ of Lamarck. And if the _Madrepora retepora_ of Solander and Ellis, is a true _Porites_, as M. Lamarck supposes it to be from the appearance of its tubes, I should conclude this genus to be very proximately allied to _Favosites_, by that species and the _F. striata_ having in common the remarkable character of fenestrated paries. But to this character I should conceive a generic importance ought to be attached, as indicating a differential organization of the artificers. I have no doubt that on close inspection of a perfect specimen, the same character will be found to exist in _F. Gothlandicum_, and possibly also in _F. truncata_, if not in the latter only, it may be proper to separate the genus and to withdraw from _Porites_ the forementioned species, retaining to _striata_ as specifically essential, the second member of the differential description. (_To be continued._) PHYSICS, CHEMISTRY, &c.