Vicarious avicularia, that is to say avicularia substantially as large as zoœcia, have rarely been figured among the Cretaceous Cribrilinidse, the only good instances I call to mind being Escharipora incrassata, D'Orb., Membraniporella (Cribrilina) Falcoburgensis, Perg., the illustration of which shows a clear case, although no reference is made to it in the text, and Cribrilina ostreicola, Bryd. They are not, however, by any means so rare as this might suggest, and the four following species are probably not the only instances which will ultimately be added from the English Chalk.

In the three preceding papers of this series descriptions have been given of the perignatliie girdles of familiar representatives of the three families into which the Holectypoida are at present divided. From the scanty evidence available, it would appear that in respect of this structure Plesiechinus is typical of the Pygasteridæ (even including Anorthopygus). The girtile of Holectypus seems essentially similar, thereby differing from that of the Discoidiinse; but as yet the perignathic structures of the Cretaceous Coenholectypus are unknown.

The detrital matter underlying the Suffolk and N.W. Essex Crags forms an incongruous mass of disrupted local rocks and fossils, supplemented by a few transported boulders of igneous and sedimentary rocks of different ages and mostly small. To these have to be added organic remains of many classes in various conditions of preservation.

The Sierra Leone, from which the Colony of Sierra Leone takes its name, is a range of palm-covered hills running parallel to the coast (N.N.W.-S.S.E.). It is truncated on the north by the wide mouth of the Roquelle (Rokell) River, which forms the magnificent harbour of Freetown. Towards the south the range terminates at Yawry Bay. The length of the Sierra is therefore about 25 miles and its width, from Kassa Town to Kissy, about 8 miles. On the east side the range descends steeply to the Kwaia plain, from which it is separated by Waterloo Creek. The detachment of the hills from the interior lowland is sufficiently complete to entitle one to speak of the Sierra Leone Peninsula. Freetown is built on the north end of the peninsula, overlooking the harbour, and it straggles up from sea-level to a height of some 800 feet on Wilberforce Hill. The greater part of the peninsula is covered with thick tropical vegetation riglit down to sea-level; and an additional obstacle to geological study is created by the heavy covering of laterite which screens the rocks from observation. In places this laterite mantle is 30 feet thick. But along the northern shore of the peninsula, from Cape Sierra Leone to the mouth of Waterloo Creek, there are nearly continuous exposures of norite, and further useful exposures have been made during the construction of the Hill Railway and along certain of the roads on the hillside. These outcrops make it clear that the whole of the north end of the Sierra is formed of norite, and it would not be surprising to learn that the entire Sierra has the same composition.