34 SUMMABY OF CUERENT RESEARCHES RELATING TO 



Tracks of Terrestrial and Fresh-water Animals.* — T. M'K. 

 Hughes describes some peculiar markings on mud, the manner of 

 formation of which he has been able to observe, and points out how 

 they explain away difficulties which have arisen in the interpretation 

 of certain fossil tracks, showing that some of the characters most relied 

 upon to prove the vegetable origin of the fossil forms, such as 

 branching, solid section, &c., could be produced by animals. 



His observations were made on certain pits in the district about 

 Cambridge which are filled with the fine mud produced in washing 

 out the phosphatic nodules from the Cambridge greensand. As the 

 water gradually dries up, a surface of extremely fine calcareous mud 

 is exposed. This deposit is often very finely laminated, and occa- 

 sionally among the laminaa old surfaces can be discovered, which, 

 after having been exposed for some time to the air, had been covered 

 up by a fresh inflow of watery mud into the pit. The author describes 

 the character of the cracks made in the process of drying, and the 

 results produced when these were filled up. He also describes the 

 tracks made by various insects, indicating how these are modified by 

 the degree of softness of the mud, and points out the differences in 

 the tracks produced by insects with legs and elytra, and by annelids, 

 such as earthworms. The marks made by various worms and larvae 

 which burrow in the mud are also described. Marks resembling 

 those called Nereites and Myrianites are produced by a variety of 

 animals. The groups of ice-spicules which are formed during a 

 frosty night also leave their impress on the mud. The author 

 expresses'the opinion that Cruziana, Nereites, Crossopodia, and Palceo- 

 chorda are mere tracks, not marine vegetation, as has been suggested 

 in the case of the first, or, in the second, the impression of the actual 

 body of ciliated worms. 



Growth of Carapace of Crustacea and of Shell of MoUusca.t — 

 A notice is here given of T. TuUberg's essay on this subject,^ in 

 which he states that the carapace of the lobster is formed by the 

 subjacent cells, the outer part of which becomes directly converted 

 into the hard covering ; the striation is due to the fibres being im- 

 bedded in the fundamental substance ; these fibres are formed by the 

 cells at the time when the enveloping substance is deposited. 



On the other hand, the shell of the MoUusca is, for the most part, 

 a secretion from the cells of the mantle, but there is, in addition, a 

 substance which in structure calls to mind the carapace of the lobster, 

 where, too, the outer part of the cells gives rise to the shell-substance. 

 The operculum of the whelk appears to be formed in the same way 

 as its shell. 



The researches have been carried on in too few species to justify 

 any general conclusions, but if we take into consideration the great 

 resemblance which obtains between all chitinous formations, it hardly 

 seems rash to suppose that they are all formed like the carapace of 



* Abstr. Proc. Geol. Soc. Loud., 1883, No. 443, pp. 10-11. 

 t Arch. Zool. Exper. et Gen., i. (1883) pp. xi.-xiv. 

 X In K. Svenska Vetens.-Akad. Handl., xix. (1882). 



