346 PROCEEDINGS OF THE ACADEMY OF [1889. 



United States," the figure given therein having been taken from 

 Dr. Gould's Invertebrata of Massachusetts, and the name C. glauca 

 replaced by that of C. fornicata. 



The speaker was not unaware of the fact that Mr. Tryon at one 

 time entertained a similar opinion regarding the affinity of the two 

 species, but a more recent examination of a large series of shells 

 convinced him that the two forms were specifically distinct. While 

 it is true that the shells of C. glauca are in a measure allied to those 

 of C. fornicata it is quite as certain that the one can be distinguished 

 from the other by at least three characters, either of which is fixed 

 and prominent enough to constitute a species. 



In C. fornicata the following permanent characters may readily 

 be observed : — 



1st. The prominent apex, which is nearly always curved laterally 

 and joined by a shelly growth to the shoulder of the shell. 



2nd. The saucer-like depression of the arch which is the same in 

 all specimens, whether young or old. 



3d. The peculiarly bowed or ogee form of the free edge of the 

 arch. 



4th. The ever-present notch at the junction of the arch with the 

 body of the shell. 



5th. The pear-shaped outlines of the shells. 



A careful examination of the shells of C. glauca will show that 

 all of these characters are absent, and that others quite as positive 

 take their place. 



The apex of C. glauca is not only less prominent but much less 

 inclined to curve laterally. It is also more acutely pointed, usually 

 shining and horn-like in appearance, and the tip is always free from 

 contact with the body of the shell. 



The free edge of the arch is also straighter than that of ('. forni- 

 cata, while, in place of the notch, which is so marked a feature in 

 the last-named species, there is a slight forward growth of the arch- 

 plate along and against the wall of the shell. But the most impor- 

 tant character, perhaps, is the elevation of the arch. This feature is 

 a constant one and sufficient of itself to separate the species from 

 C. fornicata, the arch of which is always depressed. It may also be 

 said that the two species are wider apart at maturity than at any 

 other stage of growth — although we are told that C. glauca just at 

 this period merges into C. fornicata. Other differences including 

 that of color and the quoit-like form of the shells, might be indicated, 

 but those already referred to will doubtless satisfy the student that 

 the two species are essentially distinct, 



November 19. 

 The President, Dr. Joseph Leidy, in the chair 

 Twenty-three persons present. 



