Cambridge Philosophical Society. 69 



superne aspero, caudxque aculeis uni-seriatis. Specimen minimum, 

 2-unciale. 



Among the previously described species contained in the collec- 

 tion was a specimen of Scyllium marmoratum, Benn., (Memoirs of 

 Sir T. Stamford Raffles, Appendix) hitherto only known as an in- 

 habitant of the Indian seas. 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 



Nov. 1 4. — Prof. Sedgwick, the President, in the chair. 



A memoir by Prof. Airy, was read, On some new modifications 

 in the phaenomena of Newton's rings ; of which an account was 

 given in the Phil. Mag. and Annals for August. 



The reading of a paper by Prof. Henslow, On a Hybrid Digitalis, 

 was commenced. — A lecture on the Geological Structure of Caer- 

 narvonshire was delivered by Professor Sedgwick. 



Nov. 28. — Prof. Sedgwick in the chair. 



The Rev. L. Jenyns read, A Monograph on the British Species of 

 Cyclas and Pisidium. 



The object of this paper is to present a detailed history of the 

 British species of freshwater bivalve Mollusca belonging to La- 

 marck's genus Cyclas. Only three of these shells, of which the 

 Tellina cornea of Linnaeus is one, are described by Montagu in his 

 Testacea Britannica, and not more than twice that number are to 

 be found in the latest work that has appeared on this subject. In 

 the present paper the list is extended to nine species, with the ad- 

 dition of many remarkable varieties, all of which are characterized in 

 the paper with considerable minuteness of detail, accompanied in 

 many instances by highly magnified representations. There are also 

 annexed remarks upon the nature of the animal, more particularly 

 with respect to the structure of the siphonal tube, and likewise on 

 its habits. The author follows Pfeiffer in distributing these Mol- 

 lusca under two genera, Cyclas and Pisidium, which, he observes, 

 offer points of distinction both in the animal and in the shell. In 

 the former genus the siphon is double at its extremity, and the shell 

 nearly equilateral, the posterior portion of this last being very slightly 

 longer than the anterior : in the latter the siphon is single, and the 

 shell inaiquilateral ; the anterior portion being distinctly the longer 

 of the two.— Considerable attention is also paid to the synonymy,^ 

 which was in a state of some confusion, from the circumstance of 

 the same name having been applied in several instances to more 

 than one species. 



A lecture was delivered by Mr. Whewell, On the Theory of 

 Vapour and the Formation of Clouds. —It was observed that the first 

 explanation of the ascent of vapour adopted in modern times was 

 the theory of hollovo spherules, in which the particles of water were 

 Bupposeti to exist as globular films containing air. The next was 

 the theory of the chemical solution of water in air, which, though 

 apparently untenable, still tinges our common language on the sub- 

 ject ; as when we talk of the air being saturated with vapour, of the 



precipitation 



