28 Dr. T. Williams on the Mechanism of Aquatic 



three or four adults of each kind should be thus transmitted, to 

 supply our collection with skeleton and stuffed specimens, in 

 addition to at least one to be retained entire in spirit. The 

 mici'o -mammalia, as they have been designated (as Bats, Shrews, 

 Mice, &c.), require to be thus amply represented in museums, 

 for their specific distinctions to be rightly understood in many 

 cases ; and the chaos of Indian Murid^, in particular, will be 

 never reduced to systematic order, with the synonyms correctly 

 adjusted, until such a tolerably complete collection of them from 

 all quarters has been brought together. 



[The notes appended to the present memoir were most of 

 them taken from specimens in the British and India-House 

 Museums, at the request of Mr. Blyth, and kindly permitted by 

 Dr. Gray and Dr. Horsfield ; but having reached Calcutta too 

 late for insertion, Mr. Blyth has requested me to add them to 

 his memoir, and have the whole republished in the ' Annals and 

 Magazine of Natural History.^ I have acted in accordance with 

 his wishes, and also added references to two species described 

 by Dr. Horsfield, and given the description of what appears to 

 be hitherto an un described species, in order to render it as com- 

 plete a monograph of the Indian species as circumstances would 

 permit.— U. F. T.] 



IV. — On the Mechanism of Aquatic Respiration and on the 

 Structure of the Organs of Breathing in Invertebrate Animals. 

 By Thomas Williams, M.D. Lond., F.L.S., Physician to the 

 Swansea Infirmary. 



[With a Plate.] 



[Continued from vol. xvi. p. 421.] 



Pectinibranchiata, 



This order comprehends a considerable number of families and 

 genera. It is the largest and most important group of the 

 Gasteropod MoUusks. In this summary it will be impossible 

 to present a correct analysis, derived from personal observation, 

 of the respiratoiy organs of every genus. If that were possible 

 indeed to a single observer, an acquisition of great value would 

 accrue to science. The author is deeply persuaded that even in 

 such minute constituents of the organism as a single leaflet 

 from the branchial apparatus, the microscope may reveal the 

 presence of differences of shape, size, structure, &c., which may 



