At the commencement of the 1924 geological expedition to Iceland, which was conducted by Dr G. W. Tyrrell and the present writer, aided by a grant from the Carnegie Trust, the author took advantage of his earlier arrival in Iceland by devoting his time to a detailed survey of Viðey, a small island which lies 3 miles E.N.E. of Reykjavík. His interest having been aroused by a study of the curious rock specimens which Mackenzie collected from that locality in 1810 (author, 1925), the writer intended at first to make only a rapid examination of the island in order to ascertain their field relations, and, if possible, to determine their mode of formation. A day's field work, however, convinced him that a more protracted survey would be amply repaid, and this was accordingly undertaken; during two days, out of the total of ten which were devoted to this work, the writer enjoyed the invaluable assistance of Dr Tyrrell.

Jan Mayen, so far as at present known, is entirely volcanic in origin. The rocks, therefore, are conveniently grouped with the Tertiary and later volcanics of the Faroes, Iceland, and Scoresby Sound in East Greenland. It should be noted, however, that these three localities are linked together by the Faroe-Iceland Ridge, the water everywhere being shallower than 500 fathoms. Jan Mayen, on the other hand, though lying at the same distance from Iceland as the Faroes, is not on the Ridge, and is separated by depths of over 1000 fathoms. Its position is somewhat to the west of the centre of the Norwegian Sea, and at no great distance from depths exceeding 2000 fathoms.

Benbecula signifies “the hill of the fords,” and the island was so designated owing to the fact that there is on it only one hill of any prominence, and that it is connected with the larger islands to the north and south by fords passable only at low water. The origin of the name “Uist” is referred to in a previous communication.

These islands occupy a central position in the Outer Hebrides, lying between South Uist and Harris. They are bounded on the west and north-west by the Atlantic Ocean; on the north-east by the Sound of Harris; on the east by the Little Minch, which at its narrowest part is about 16 miles broad; and on the south by a narrow channel separating Benbecula from South Uist. The area embraced in this communication extends from the Isle of Shillay in the north to the Isle of Wiay at the south-east corner of Benbecula, and includes not only the main islands of North Uist and Benbecula but also a number of smaller isles adjoining the main ones. Including the intervening channels, the total length from Shillay in the north to the southern extremity of Benbecula is about 28 miles.

Jan Mayen is so inaccessible and so infrequently visited that very little geological and petrographical work has been done on the island. Some rocks were collected during the Austrian “Pola Expedition” of 1882–3; and these were described by Scharizer as porphyritic olivine basalts, with one example of ægirine-trachyte. Four analyses were given, but their quality is so poor that they are practically useless for scientific discussion. Washington lists only one of these, and that among the analyses of inferior quality. The only modern petrographic work that has been done on Jan Mayen rocks is the description of an olivine-trachyandesite by Dr Holmes accompanied by a first-rate chemical analysis by Dr H. F. Harwood, which forms a valuable addition to the seven new analyses recorded in this paper.

Whatever may be the worker's views on the origin of the Stegocephalia, and on the relation of the various divisions of the Pisces to one another and to the Amphibia, he cannot but agree that a knowledge of the anatomy and of the embryology of the Polypterini is of paramount importance to all those whose research or teaching is concerned with the evolution of the Vertebrata. So he will understand my pleasure when, through the kindness of Dr F. F. Bond, I was able to obtain from Old Calabar a number of specimens of the less-known genus of the Polypterini, Calamoichthys, which were so well preserved as to be excellent for dissection and, as a whole, quite good for histological detail.

In a preceding paper of this series it was pointed out that the smooth branched axes, which are so common in the Middle Old Red Sandstone of Scotland, cannot be assumed to belong to one kind of plant. They were therefore for convenience referred to as Hostimella sp. It was further shown that three distinct types of sporangia were associated with axes of this general type and, on the characters afforded by the reproductive organs, three species were distinguished. These were provisionally retained under the generic name Hostimella, as H. pinnata, H. globosa, and H. racemosa. In the same paper it was mentioned that the linear bodies borne on the peculiar fossil known as Ptilophyton Thomsoni, Dawson, were almost certainly sporangia. This afforded another reproductive structure borne on a vegetative region, which, if found separately, could only be placed in Hostimella sp. Since, as had been pointed out by Kidston, the generic name Ptilophyton is inadmissible for this fossil, it was placed in the purely provisional genus Hostimella as H. Thomsoni. In the present paper the nature of this interesting member of the Scottish Middle Old Red Sandstone Flora will be more fully considered.