178 J. D. Dana — Bamming of Streams hy drift ice 



In the description it is said : " Muscular impressions in the 

 ventral valve, four; one pair in front of the beak, near the mid- 

 dle or in the upper half of the shell." The pair here alluded to 

 are the laterals. Their upper and lower extremities are some- 

 times not visible, and what remains occupies the middle por- 

 tion of the length of the shell. The expression "or in the 

 upper half," I can thus explain: I had the dorsal valve of 

 0. crassa^ from Troy, which I then supposed to be a ventral 

 valve. In this the laterals are in the " upper half." The trans- 

 verse scars were not then observed and hence four scars instead 

 of six. It must be borne in mind that fourteen years ago noth- 

 ing was known of the internal characters of these shells. The 

 materials were imperfect and consequently so was the descrip- 

 tion. It is now certain that the genus is a good one and that 

 all of the three species on which it was founded belonged to it 



The described species which I consider to be truly within the 

 genus are : 0. chromatica, 0. polita, 0. crassa, 0. nana, and 0. 

 gemma. They all, so far as is yet known, are confined to the 

 Potsdam Epoch. A number of other species have been referred 

 to the genus, but they are all more or less doubtful. 



The specimens which have furnished the above additional 

 details of the structure of 0. chromatica were collected at 

 L'Anse au Loup, the only place where the species has been 

 found, in 1863, by T. C. Weston of our Survey, and by him 

 very skilfully worked out of the matrix. 



Art. XXL— On the Damming of Streams hy drift ice during the 

 melting of the great Glacier ; by J. D. Dana. 



When treating of the overflows of the flooded Connecticut, 

 in the Supplementary December Number of this Journal, (p. 

 497,) I suggested, in view of the fact that the terraces in the 

 Farmington Valley about Tarifville and Simsbury are at least 

 50 feet higher than those a mile eastward in the parallel Con- 

 necticut valley — that the gorge through the Divide Range, by 

 which the Farmington river there passes into the Connecticut 

 valley, had been closed by drift and so remained until the flood 

 had reached its height. 



I allude to this subject again to add that the events connected 

 with the opening, in the Spring, of many of our modern ice- 

 covered streams aiford abundant reason for believing that, 

 during the breaking up of the long Glacial winter, when the 

 ' y forward, the gaps, gorges or narrows, along 

 1 liable to obstruction by 



