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S U M M A R YThe amount of leaf litter buried by worms between leaf fall and spring was measured in two stages in five grass and two arable orchards near Wisbech, Cambs. A known number of weighed leaves was placed on the ground beneath wire cages at leaf fall and again several weeks later and those remaining at the end of each stage were counted and weighed. Unburied leaves lost about one-quarter of their dry weight during each stage. Buried leaves were pulled into burrows by Lumbricus terrestris (Linn.), apparently the only species present that did so. Populations of L. terrestris beneath the cages were estimated from the number brought to the surface by dilute formalin.In grass orchards, the number and weight of leaves buried was closely correlated with the estimated fresh weight of L. terrestris. Some orchards had a to I ton of L. terrestris per acre and in these over 90 "/b of the normal leaf fall (3 ton dry weight) was buried during winter. Earthworm activity could account for the observed amounts of leaf litter removed in winter and observed differences between orchards could be attributed to differences in their L. terrestris population.Apple leaves were buried more rapidly in arable than in grassed orchards, probably because they were the only ones and were easier to find and bury when there was no grass. Evidence to support this came from one grass orchard where the number of leaves buried from each cage was inversely proportional to the amount of surface vegetation. Twice as many leaves were buried in cages initially containing zoo leaves as in cages containing IOO leaves.The rate at which L. terrestris buried leaves depended on soil temperature, and burial probably stops at 2 ' C., which is about 1.6" C. below the lowest weekly average temperature reached in Wisbech soils in winter.The long-term effect of leaf burial was shown by the contrasting soil profiles of a typical grass orchard and one where L. terrestris is rare, where the crumb structure is poor and where the accumulated remains of up to 15 years leaf fall now form a surface mat 4-14 in. thick.

I. Introduction. The above-named species of Olenid was first recorded in 1874 by Callaway from the Shineton Shales of Shropshire, which he then claimed as of Tremadoc age. It was described from imperfect material as a Conocoryphe , which name in 1877 he corrected to Olenus , giving at the same time a description of the entire trilobite. A specimen for the first time exhibiting dorsal spines was in 1900 described by Dr. H. H. Thomas under a new name, Olenus mitchinsoni . An additional occurrence of the species was in 1905 recorded by Prof. W. G. Fearnsides under the name of ‘ Parabolinella salteri ’. It occurred in the upper part of his Nant-ddu or Bellerophon Beds, which follow next above the Dictyonema Band north-west of Arenig Fawr, and it is there accompanied by Asaphellus homphrayi Salter and Parabolinella triarthrus (Callaway), which species are also associated with it at Shineton. In 1907 I briefly described its development, and assigned it to a new sub-genus ‘ Leptoplastides ’. Finally, Mr. P. Lake redescribed it in 1919, under the name Leptoplastus salteri . It is now many years since, at Prof. Charles Lapworth's suggestion, I undertook the study of the trilobite fauna of the Shineton Shale. The first collection studied was that collected by Mr. Rhodes for H.M. geological Survey, and lent to Prof. Lapworth by the then Director-General. Among these were several slabs covered with minute examples of the fossil in question, in different stages of development; and the early stages seemed so strange and unexpected that a special study

The trilobite family Mesonacidae was erected by Walcott (1890 b ) in his “Fauna of the Lower Cambrian or Olenellus Zone” for the genera Olenellus, Mesonacis , and their relatives, which especially characterize that rock series. The first discovery of a Mesonacid in Britain was made by C. Lapworth at Comley in Shropshire; and was recorded in 1888 in the Geological Magazine and in Nature , the species being named Olenellus callavei . In 1890 a short characterization of this species by him appeared in Walcott's above-mentioned monograph (p. 640), and Lapworth himself in 1891 described and figured the species under the name Olenellus ( Holmia ) callavei . In 1910 appeared Walcott's monograph on “Olenellus and other genera of the Mesonacidæ”. In this, Lapworth's species was illustrated and was referred to Matthew's genus Callavia , to which also was referred a new species from Comley, the characteristics of which were furnished by Raw, described below (p. 274) under Nevadia (Walcott, 1910, pp. 282–3, pl. 42, figs. 1–4). In this monograph were brought under review all the known examples of the family, many of them discovered and described by Walcott himself; and to it as well as to his previous and later papers all students of the family are indebted. Earlier than this Dr. E. S. Cobbold had commenced his intensive study of the Cambrian rocks and fauna of Shropshire, furnished not only in natural exposures and in quarries, but also by about sixty special excavations made by him, about half of which exposed Lower Cambrian rocks. In a

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