Barrettia, the most abnormal of the Rudistæ, and among the strangest of all the mollusca, was first found in Jamaica by Lucas Barrett, F.G.S., in the year 1861. The specimens were ably described by S. P. Woodward, F.G.S., in 1862. The original specimens, now in the British Museum of Natural History, consist of polished transverse and longitudinal sections of the upper part of the shell, the latter showing the junction of the upper and lower valves, and a cross section of the young shell near the apex of the lower valve. They were called Barrettia monilifera in honour of the discoverer. The locality where they were found was on the banks of the Back River, a tributary of the Rio Grande, about 15 miles from the coast in Portland Parish in the north-east of the island. The rock is described as a hard, grey limestone which occurs in bands of a few inches to a yard in thickness, subordinate to many hundreds of feet of shale which graduate up into other grey shales of the Eocene, followed by white limestones of Miocene age.