82 REPORT — 1851. 



Some of the specimens found are the most beautiful of the British Lepralias, one new 

 to the British list, found also by him in Cornwall and Scotland ; he also produced one 

 new to the Britisli list from Cornwall, identical with a Lepralia on a Pinna from the 

 Mediterranean. 



He also called attention to the scooping of pits in shells and stones by Lipralia and 

 other calcareous zoophytes, and exhibited several specimens of shells and a stone which 

 had been scooped by one of the most delicate of them, Hippothoa divaricata, which it 

 had equally scooped with those of larger size. Some of the shells had not only the 

 pits, but the cells of Lepralia as well. 



Observations on the Geographical Distribution of the Land Mollusca. 

 By LovELL Reeve, F.L.S. 

 This paper consisted of a few additional observations on the geographical distri- 

 bution over the globe of a tribe of snails, recently published by Mr. Reeve in the 

 'Annals and Magazine of Natural History.' It was doubted at the time whether a 

 system of typical arrangement could be formed upon the consideration of a single 

 genus, — whether an examination of all the genera of land mollusca was not essen- 

 tial. It being a work of considerable labour to collect the data for this inquiry, 

 the author submitted a preliminary outline of his views from a consideration of 500 

 Bulimi only. He was now working upon the genus Helix, andfound the results to be 

 perfectly similar. The inquiry was founded on a considei-ation of the shell, not on ac- 

 count of the difficulty of procuring observations on the animal, but because it offered 

 a readier and more varied set of characters. The animal differs immaterially in form 

 and generic character from any of the genera of land mollusca, but the shell varies ac- 

 cording to the physical conditions by which it is surrounded. The groups of forms 

 appear to be so many expressions of the calcifying organ of one and the same mollusk, 

 depending on the laws and circumstances of organic distribution. On comparing his 

 map of the distribution of land mollusca with that of the marine mollusca by Professor 

 E. Forbes, Mr. Reeve pointed out many points of resemblance where the areas of sea 

 distribution of types corresponded with those on the adjacent land. 



Observations on Pholas. By J. Robertson. 



On the Structure of the Branchice and Mechanism of Breathing in the Pholades 

 and other LamelUbranchiate Mollusks, By Thomas Williams, M.D. 

 The researches of the author, which were illustrated by numerous diagrams, had 

 led him to the following conclusions : — 



1. That the blood in all lamellibranchiate mollusca is richly corpusculated. 



2. That the branchiae in all species are composed of straight parallel vessels 

 returning upon themselves. 



3. That the heart is systemic, and not branchial. 



4. That the parallel vessels of the gills are provided with vibratile cilia disposed in 

 linear series on either side of the branchial vessel, causing currents, which set in the 

 direction of the ciu-rent of the blood in the vessels. 



5. That in Pholas the siphons are richly lined with vibratile cilia as well as the 

 branchiae. 



6. That the branchial siphon acts in drawing in water into the chamber of the 

 mantle by the diastole of the valves of the shell. 



That a part of the water which is thus drawn into the branchial chamber, is swal- 

 lowed and eventually rejected by the faecal orifice, and that the rest is expelled by 

 the orifice in the mantle, the foot, and in part by the branchial orifice. 



That this respiratory fluid is surcharged with carbonic acid and fluid secretions 

 contained in the mucus furnished by the interior of the mantle. 



That this current, escaping with force against the walls of the cell in which the 

 animal lives, acts as a solvent upon the particles disintegrated by the action of the 

 valves ; that therefore the boring of the Pholades can only be explained on the prin- 

 ciple that a chemical as well as a mechanical agency is at work. 



