1871.] 153 [Morse. 



lower half, the successive accretions being produced around the 

 byssus. This growth for a while seems to take place exclusively 

 from the posterior half of the shell limited in front by the byssus, 

 and even after this growth has increased to twice the diameter of 

 the embryo shell only a slight increase is noticed on its anterior mar- 

 gin, this latter addition being slightly reflected. 



The left or upper valve grows more rapidly, so that its margin 

 overlaps the right valve at all points. 



As the animal increases in size, the foramen increases also, and its 

 earlier boundaries are consequently absorbed. 



It will be seen by reference to the figures that the growth of the 

 perforated valve is first posterior and downward, from the posterior 

 half of the shell; it then grows forward, avoiding the byssal plug, 

 and by successive additions surrounds the byssal plug and ulti- 

 mately reaches the umbones of the larval shell, and even beyond 

 and behind this region. From these facts it is obvious that at an 

 early stage the animal is free, and for a time locomotive; that it has 

 an elongate, oval, bivalve shell, with close and regular lines of ac- 

 cretion, and that during the latter stage of this growth it becomes at- 

 tached by a byssus passing from between the valves as in Mytilus; 

 that before the growth of the larval shell is completed, it drops over 

 to one side, since one valve only shows the notch upon its margin, 

 and that so soon as this growth ceases a new growth takes place, 

 looser in texture, and white in color, as above described. In this ex- 

 cessive growth of the shell backward it is interesting to note that in 

 the Mytilidas the growth of the shell is almost entirely posterior, 

 leaving the umbones at the extreme anterior portion of the shell. 



It would appear that the larval shell is a true dimyarian and 

 its affinities may possibly be quite remote from Ostrea or Pecten. A 

 study of the early stages of the last named genera would easily 

 remove all doubts upon these points. 



Drs. J. B. S. Jackson and B. Joy Jeffries, and Messrs. R. 

 C. Greenleaf, John Cummings, Jr., and Wm. H. Niles, were 

 elected a Committee of Nomination, to present the names of 

 candidates for the officers of the Society for the ensuing year. 



Mr. Sanborn exhibited a valuable donation of alcoholic 

 specimens of mammals, birds, fishes, reptiles, mollusks and 

 insects from Florida, presented by Mr. Samuel N. Chamber- 



