THE LAMELLIBRANCHIA 253 
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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 T'ridacna, 
some specimens of the latter genus weighing as much as 310 lbs.). 
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 Luciroa (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 Ridewood, 
undertaken at the instance of Ray Lankester, have shown that 
it was the objections of Dall that had no foundation: Hucivoa 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 
