KIXETOGENESIS. 261 



Opisthobranchs, Mar^eniidae, Sigaretus, Harpa, etc.). 

 In a group like the Cypraeidae, where nearly all the 

 species are lirate on both lips, there are a few which 

 want these lirae, and these are species which have a 

 wider aperture in the adult than most of the genus, 

 and in which we should expect the wrinkles to be less 

 emphatic." 



b. Mechanical Origin of Characters in the Lamelli- 

 branchs (^Pelecypodd). 

 Dr. Robert T. Jackson has pointed out^ the history 

 of the characters of the retractor muscle and some of 

 those of the list, of bivalve Mollusca. I take the fol- 

 lowing abstract of his conclusions: "In the develop- 

 ment of pelecypods we find in a late embryonic stage 

 (the phylembryonic) that the shell has a straight hinge- 

 line. This is characteristic of Ostrea (Fig. 59), Car- 

 dium, Anodonta, and so many widely separated genera 

 that it apparently represents a primitive ancestral con- 

 dition common to the whole class. Embryology shows 

 that the bivalve shell doubtless arose from the split- 

 ting on the median line of a primitive univalvular an- 

 cestor. If that ancestor had a saddle-shaped'^ or a 

 cup-shaped^ shell, as is probable, the first result of the 

 introduction of a hinge in the median line would have 

 been to straighten the shell on the hinge-line. This 

 is a simple problem in mechanics, for if one tries to 

 break by flexion a piece of metal which is saddle- 

 shaped or cup-shaped, it will tend to form a straight 

 line on the axis of flexion. A parallel case is seen 

 in the development of a bivalve shell in ancient crus- 



\ Memoirs of the Boston Society of Natural History, Vol. IV., No. 8, p. 277 

 July, 1S90; American Naturalist, 1S91, p. ii. 



2 Characteristic of young Dentalium. 



SCharacteristic of the extreme young of ceplialou:; Uculiiisks. 



