THE LAMELLIBRANCHIA 253 



extent swim by forcibly expelling water from the posterior aperture 

 of the mantle (Solen, Solenomya). 



In point of size the Lamellibranchs vary from a length of a few 

 millimetres to more than seventy centimetres (Pinna and Tridacna, 

 some specimens of the latter genus weighing as much as 310 Ibs.). 

 The fossil Hippurites attained to the length of a metre. 



There are more than 5000 living species of Lamellibranchia, of 

 which 1000 are Unionidae. They are distributed all over the 

 world, and some marine forms extend to a depth of 2700 fathoms. 

 Fossil forms appear in the Cambrian, and become very numerous 

 in species from the Silurian onwards. Some large groups, such as 

 the Palaeoconcha of the primary and the Rudistae of the secondary 

 deposits, are quite extinct. 



V. REVIEW OF THE ORDERS AND FAMILIES OF LAMELLIBRANCHIA. 



The classification of this homogeneous group has long presented 

 great difficulties, for the different organs or apparatus, such as shell, 

 muscles, siphons, etc., that have successively been employed as 

 bases of classification, have not given satisfactory results. 



Ray Lankester was the first to suggest (in 1884) that the 

 structure of the gills might furnish characters of classificatory 

 value, and the present writer has constructed on this basis a 

 phylogenetic classification in which the class is divided into five 

 groups. This classification has put various families, such as the 

 Anomiidae, Trigoniidae, Dreissensiidae, etc., into their proper 

 places, and has been largely adopted. Objections to it have, 

 however, been raised, notably by Dall, who has urged that the 

 genera Eudroa (Anatinacea) and Callocardia (or Vesicomya, 

 Cyprinidae) have protobranchiate gills, and that the system of 

 classification according to branchial characters is consequently 

 without foundation. But the recent investigations of Ride wood, 

 undertaken at the instance of Ray Lankester, have shown that 

 it was the objections of Dall that had no foundation : Eudroa and 

 Callocardia have typical eulamellibranchiate gills. 



As the result of the advancement of our knowledge, the 

 classification of the Lamellibranchia founded on the structure of 

 the gills has been ameliorated by the suppression of the order 

 " Pseudolamellibranchia," and the two diphyletic sub-orders which 

 it included, the Pectinacea and the Ostraeacea, may be respectively 

 located in the Filibranchia and the Eulamellibranchia, thus making 

 these two old-established orders correspond to the new orders 

 proposed by Ridewood under the names Eleutherorhabda and 

 Synaptorhabda. 



On the other hand, the shell (and particularly its hinge) is the 

 only other organ that has been retained as a basis of the general 



