The structure of the anterior part of the alimentary canal of stenoglossan prosobranchs has been previously investigated by several workers, of whom the most important are Haller (1888) and Amaudrut (1898), although Haller's account, accurate so far as it goes, was published before there was any real knowledge of the comparative morphology of that part of the prosobranch gut. This knowledge we owe to Amaudrut, but in the case of the stenoglossan œsophagus the description which he gives is wrong, the various parts being orientated upside down.

The statistical material of genetics usually consists of frequency distributions—of genes, zygotes and mating couples—from which new distributions referring to their progeny arise. Combination of distributions by random mating is usually symbolised by the mathematical sign for multiplication; but this sign is not taken literally for the simple reason that the genetical laws connecting the distributions of progenitors and progeny are inconsistent with the laws governing multiplication in ordinary algebra. This is explained more fully in § 2.

1. Tulach Hill (1537 ft.) rises steeply from the southern side of the River Garry, about a mile to the south-west of Blair Atholl (fig. 1). It is the culminating point of a low range of hills running north-west from the confluence of the rivers Garry and Tummel and lying between Strath Garry on the north and Strath Tummel and Fincastle Glen on the south. To the west, the Tulach ridge descends in a drift-covered slope into the valley of Allt Bhaic, a northward flowing tributary of the Garry with its source in Loch Bhac.

The medusa Cosmetira pilosella Forbes is known, from the work of Miss M. J. Delap, to arise from a Cuspidella hydroid (Russell, 1936). Recently I found a small fertile colony of a Cuspidella on a dead bivalve shell dredged off Brigurd Buoy in the Firth of Clyde and this later liberated young medusæ in the laboratory. These were identified as Cosmetira pilosella, thus confirming that the hydroid of C. pilosella is a Cuspidella. There is no previous description of the hydroid or of the newly liberated medusa and so a description is given below.

Among the genera of fern-like plants of Palæozoic Age there are a number which have orbicular or reniform leaflets. Detached leaflets of this form are frequently found, and it is a matter of difficulty to determine to which of several genera such fragments belong.

Under the name Cyclopteris Brongniart (1828, p. 215) described some orbicular leaflets found in Upper Carboniferous strata, and later investigators have shown that most of these leaflets were the basal or stipular pinnules (aphlebiæ) of the frond of the Pteridosperm Neuropteris. They were sessile on the rachis and had broad bases of attachment, while the shape of the pinnules on the upper parts of the frond was quite different. The Scottish fossils which will be described in this memoir are also orbicular leaflets and have been named Cyclopteris by some authors. There is evidence, however, that they were pedicillate and were not the aphlebiæ of the frond which bore them.

In a recent paper (Mackenzie and Muller, 1940) a brief account was given of certain methods employed to obtain a favourable mutation rate in male adult Drosophila. These methods were largely empirical, and a detailed account of their development, together with a discussion of their significance, seemed rather beyond the scope of the paper, which dealt principally with the possible chromosome rearrangements which could be brought about by ultra-violet light. Hence only the average of a large number of experiments was given. In this paper it is proposed to give a fuller account of the influence of ultra-violet rays on Drosophila, and to compare the somatic effects, which are directly lethal, with the spermatic effects, which occur in the surviving insects and are observed as mutations in the offspring.

A cytological study of the testes of the cat was undertaken in order to disclose the structural differentiation of the sex chromosomes. It was hoped that an analysis of these particular chromosomes in the male sex during meiotic division might reveal structural peculiarities which would throw some light upon the highly complex genetical behaviour of tortoise-shell cats.

The problem of viability and fertility of interspecific hybrids in Drosophila has been attacked most recently by Muller and Pontecorvo (1941). They crossed diploid Drosophila simulans males with triploid melanogaster females, and, by using a specially designed technique, succeeded in introducing into the marked genotype of melanogaster some chromosomes of simulans. Amongst the many interesting results obtained, it was found that not all interspecific combinations of chromosomes were viable; the large chromosomes of one species show a definite “need” for some genes or group of genes in other chromosomes of the same species, and these authors have drawn the conclusion that there are a number of complementary “lethals” in both species located in the major chromosomes (X, II, and III).

A cytological analysis of a number of Sealyham, Spaniel, and Spaniel × Manchester Terrier dogs was undertaken in order to determine the number and structure of chromosomes during both mitosis and meiosis in each breed, and to ascertain the differences between them in respect of chromosome number and behaviour.