66 Cambridge Philosophical Society. 



with the greater part of the black colouring matter of the eyes, be- 

 comes fixed and covered with calcareous matter, and is changed 

 into a young Barnacle, such as is described by Pennant as Balanus 

 pusillus, the arms at the same time acquiring the usual ciliated ap- 

 pearance. In Mr. Gray's specimens of the young, on the contrary, 

 the general form of the adult is found, and the arms are ciliated 

 while it is still in the egg, its total length being less than -^th of an 

 inch. Of this length it is also by no means uncommon to find 

 common Barnacles attached. 



Mr. Gray added that on examining the eggs which are found 

 around the base of the animals of Pentalasmis, Leach, and Otion, 

 Ej., he had observed indications of the existence of young similar 

 to the adult. They were not, however, sufficiently developed to 

 enable him to describe them with precision. 



Mr. Gray also called the attention of the Society to a fact con- 

 nected with the history of some of the marine Gasteropodous Mol- 

 lusca, which he had observed on the same occasion with the young 

 of the Balani. It is well known that the animals of terrestrial shells 

 are torpid during the winter in cold and temperate climates, and 

 during the dry season or summer in tropical regions; but it had not 

 been previously remarked that a similar state occurs in those of 

 marine shells. Mr. Gray found that many individuals of Littorina 

 petrcea, and some of Litt. rudis, were in this condition during his 

 stay at Dawlish. They were attached to the rocks several feet 

 above the reach of the highest autumnal tides ; their foot was en- 

 tirely retracted ; and a membranous film was spread between the 

 rock and the edge of the outer lip of the shell : the gills were only 

 moist, the branchial sac being destitute of that considerable quan- 

 tity of water which exists in it in those of the same species which 

 are adherent to the rock by their expanded foot. In this torpid 

 condition, the individuals observed by Mr. Gray continued during 

 the whole of his stay, which lasted for more than a week. On re- 

 moving several of them and placing them in sea water, they re- 

 covered in a few minutes their full activity. 



Mr. Gray further stated that he had on the same occasion ob- 

 served that the animal of Rissoa parva has the power of emitting 

 a glutinous thread, by which it attaches itself to floating sea-weeds, 

 and is enabled, when displaced, to recover its previous position. A 

 similar property, he remarked, was long since observed in one of the 

 land Mollusca, a species of Limax, Linn. ; and it has recently been 

 recorded by M. Sander Rang as occurring in a marine genus of 

 Mollusca, to which he has given the name of Litiopa. Mr. Gray 

 added his belief that it would probably be found to be common to 

 many species of marine Mollusca. 



PHILOSOPHICAL SOCIETY OF CAMBRIDGE. 



A Meeting of the Cambridge Philosophical Society was held on 

 Monday, November 25, the Rev. Dr. Clark, V.P., being in the chair. 



Mr. Hopkins gave an account of some important points in the 

 geology of Derbyshire. The toadstone beds of the limestone district 



