Marketable securities at June 30, 2019 consisted of the following:

 

Marketable securities at December 31, 2018 consisted of the following:

 

U.S. Treasury securities include government debt instruments issued by the U.S. Department of the Treasury. Corporate debt includes obligations issued by investment-grade corporations, and may include issues that have been guaranteed by governments and government agencies. At June 30, 2019 and December 31, 2018, the Company held only investments that have maturities of less than one year.

At June 30, 2019 and December 31, 2018, the Company held 2 and 21 debt securities, respectively, that individually and in total were in an immaterial unrealized loss position for less than one year. The aggregate fair value of debt securities in an unrealized loss position at June 30, 2019 and December 31, 2018 was $1,839,053 and $18,551,296, respectively. The Company evaluated its securities for other-than-temporary impairment and considered the decline in market value for the securities to be primarily attributable to current economic and market conditions. It is not more likely than not that the Company will be required to sell the securities prior to the recovery of the amortized cost basis. Based on this analysis, these marketable securities were not considered to be other-than-temporarily impaired as of June 30, 2019 and December 31, 2018.