416 PROCEEDiiras OF THE GEOLOGICAL sociETy. [May 24, 



treidce, Mactrldce, aad Pholadidce, commence their ranges in the 

 Carboniferous Limestone. Of these six families, four (Veneridce, 

 Lucinidie, Mactridce, and Pholadidce) attain their maximum deve- 

 lopment in the Tertiaries, and two (^Pectinidoe and Ostreidce) in 

 Jurassic strata. 



The small family Gastrochcenldce, of which but few species have 

 been found in British strata, has its earliest representative in the 

 Lias, and its greatest number of species in Tertiary strata. 



The fluviatile Cyrenidce also range from the Lias, and, like the 

 Oastrocli'-enidce, have their maximum development in the Tertiaries. 



The remaining two families, Chamidce and Hipjmntidce, are nu- 

 merically unimportant, and yet are structurally perhaps the most 

 remarkable of all the Lamellibranchs, some of the forms of Chamidce 

 strongly resembling Grasteropods, and the Hippuritidce being so ab- 

 normal in form and structure as to have been for a long time con- 

 sidered a separate order, the Rudistes of Lamarck. The Hippuritidce 

 are also of great interest, as being the only extinct family of the 

 Lamellibranehiata as yet discovered. They are represented in the 

 British Islands by only one species, the Hippurites, or Badiolites 

 Mortoni of Mantell ; and this species is confined to the Upper Green- 

 sand and Chalk. 



Though we are able somewhat satisfactorily to divide the Lamel- 

 libranehiata into families, there are not suificiently good distinctions 

 for ordinal divisions. The fact of some genera, however, having 

 only one adductor muscle has been seized as a basis on which a 

 division of the class into the orders, groups, or sections Monomyaria 

 and Dimyaria has been made. This division, though perhaps not a 

 sufiiciently scientific or philosophical one, is in some respects con- 

 venient, and it has been adopted by several authors. 



A better division of the Lamellibranehiata than into Monomyaria 

 and Dimyaria is perhaps that founded on the possession, by a large 

 number of genera, of respiratory siphons, and some being unpro- 

 vided with those organs. This great structural diff'erence naturally 

 divides the class into AsipJionida and Siphonida — the SipTionida, or 

 those furnished with respiratory siphons, including by far the greater 

 number of genera. These two great sections of the class have an 

 equally extended range ; but the AsipJionida have their greatest de- 

 velopment in Jurassic rocks, while the greatest number of species of 

 SipTionida have been furnished by Tertiary strata. It may be men- 

 tioned, however, with respect to the former section, that although 

 the Jurassic system, which includes all beds from the Lower Lias to 

 the Purbeck, yields a greater number of species of AsipJionida than 

 any other of the great groups or systems of rocks, yet when we 

 compare formations we find that no single formation equals the Car- 

 boniferous Limestone in its yield of species of this division of the 

 Lamellibranehiata. (See Table II.) 



The distribution of the entire class now demands our attention ; 

 and disregarding generic, family, ordinal, or sectional divisions, and 

 having regard only to species, we find the Lamellibranehiata spa- 

 ringly represented in the lower di'^dsion of the Lower Silurian group ; 



