162 MORSE, CLASSIFICATION OF MOLLUSCA 



XL A Classification of Mollusca, based on the "Principle 

 of Cephalization." By Edward S. Morse. 



With a Plate. 



[Communicated June 19, 1865.] 



After becoming acquainted with the perfect unity of plan 

 in the Hadiata and the connected series of homologies, run- 

 ning through the whole branch, (as demonstrated oy Prof. 

 Agassiz in his private lectures) my interest was excited, to 

 discover, if possible, a like symmetry of development in 

 the Mollusca. Finding the universality of vertebration 

 among the Vertebrata, of articulation among the 

 Articulata, and similarly of radiation among the Ra- 

 diata, I could not but believe that in the Mollusca some 

 plan lay hidden, which, when unfolded, would as definite- 

 ly convey their type, and unite them alJ, as in the other 

 branches. It is not enough to call them soft bodied ani- 

 mals ; for in considering their shell as a part of their organ- 

 ization, we have among them many of the hardest ani- 

 mals known, and we also have an equal number of soft 

 bodied animals in the other branches. Their bilaterality, 

 as expressing anything definite, is an equally unsatisfacto- 

 ry character. Prof. Huxley has given an archetype, or com- 

 mon plan of the Mollusca, as he conceives it, with many 

 truthful homologies, in the article "Mollusca," English Cy- 

 clopedia, Vol. III., p. 855. In his figure of the archetype how- 

 ever, which is bilaterally symmetrical, we have details of 

 structure only. 



Prof. Agassiz in his "Methods of Study in Natural History" 

 also suggests his idea of the plan, or structure, when he 

 says, p. 34, "Right and left, have the preponderance over 

 the other diameters of the body," and says furthermore, 

 that collectors unconsciously recognize this in the arrange- 

 ment of their collections. "They instinctively give them 

 the position best calculated to display their distinctive 

 characteristics, and to accomplish this they necessarily 

 place them in such a manner as to show their sides." 

 This can refer only to the Lamellibranchs, and their shells 

 are displayed on the sides, because they naturally fall in 

 that position. This lateral preponderance of structure on- 

 ly obtains among the Lamellibranchs. All Brachiopods 



