114 Mr. M. C. Marcli on the 



Neumeyer's seven orders bear no distinct relationship to 

 the orders established by those who followed the differentia- 

 tion of the gills. Ball's first order, Prionodesinacea, corre- 

 sponds to the Protobranchia and Eleutherobranchia of 

 Ridewood, except that Ostrea and Pinna are removed by the 

 latter, on account of their gills, into the Sj'^naptorhabda, which 

 is equivalent to the Anomalodesmacea and Teleodesmacea of 

 Uall, with the exception of the above-mentioned families. 



Bernard's Pleurodonta inckides the members of the Proto- 

 branchia and Eleutherorhabda, togetlier witli the Ostreid;e 

 and the Pinnidte placed in it, and without the Cardiniidie 

 and Trigoniidse. His Heterodonta agrees with the Synapto- 

 rhabda with these two families removed and the Trigoniidic 

 and Cardiniidse added. 



Grobl)en's classification appears to be untenable for three 

 reasons : — 



A. He separates the Desmodouta from the Heterodonta, 



placing them in a different order, although they are 

 essentially similar in both the gills and the hinge. 



B. He separates the Arcidse from the other Taxodonta, 



placing them in the same order as the Heterodonta, 

 although they differ in development and history. 



C. In spite of the same difficulty, he places the Aniso- 



myarians with the Heterodonta. 



As Ball's orders have been shown to have been founded 

 on a misconception of the value of the teeth, the only 

 important comparisons are betw^een Bernard^s and Ridewood's 

 classifications. 



Oqc of the differences between these classifications is 

 the inclusion in the first of Bernard's orders of the first two 

 of Ridewood's orders. Bernard's reasons for putting the 

 Taxodonta and Dysodonta together are : — 



1. They have a similar prodissoconch with embryonic 



crenulations. 



2. The early dissoconch stages are similar in regard to the 



development of the teeth and cardinal plateau. 



The differences in their later development justifies their 

 separation into suborders. 



Ridewood's reason for separating them is that tlie gills 

 in one case are simple protobranchs and in the other they 

 are recurved. Ridewood himself derives the filibranch type 



