165 



(jijclas elegans Adams. B. J. N. H. in. 3.'}0, pi. 3, f. 11. 



Cydas minor Adams. B. J. N. H. iv. 39, pi. 4, f. 2. 



Cyclas nitida Adams. B. J. N. FI. iv. 39, pi. 4, f. 3. 



Cydas partumeia Say. Gould's Report, 73, f. 54. 



Cydas securis Nobis. 



Cydas similis Say. Gould's Report, 72, f. 53. 



Cydas truncata Linsley, Gould, Am. Jour,, 1848. 



Cydas tenuis Nobis. 



Pisidium compressum Nobis. 



Pisidium duhium Gould. Agassiz's Lake Superior, 345. 



Pisidium ferrugineum Nob is . 



Pisidium variahile Nobis. 



Pisidium ventricosum Nobis. Bost. S. N. H. Proc. iv. 68. 



Pisidium zonatum Nobis. 



Prof. Wyman said, that after the recent storm, he had 

 found on Chelsea Beach numerous eggs of the Skate, con- 

 taining the embryo at different stages of development. He 

 had also found them in the spring, showing that there are 

 two broods in a year. The same is true of the Dog fish. 



Within the general envelope of the egg, the embryo, however 

 small, was in no instance contained in any investing membrane. 

 The yolk varied much in size, a small yolk being in some in- 

 stances attached to a large embryo, and vice versa. At one 

 end of the parallelogram-shaped egg was a transverse fissure, 

 which opened to allow the escape of the young fish when fully 

 formed. This has been described as an opening for the admis- 

 sion of water to serve the purposes of respiration for the embryo. 

 The true openings for this purpose are the orifices at the ends of 

 the tubular prolongations, two of which project from each end of 

 the egg. 



Mr. Desor said, that in studying the development of 

 Eolis and Doris, of which it was not uncommon to find 

 four or five yolks in one shell, he had frequently observed 

 one yolk much smaller than the others, but the individual 

 to which it was attached was always proportionate to it in 

 size. 



