America

Rav Sonderling was a Field Rabbi in the German army before immigrating to the United States in 1923.  He co-wrote a siddur in the 1940's.  None of the authors of the siddur wore kippot.

Rav Risikoff immigrated to the United States in 1906.  He seems to conform to the trend that the only rabbis in America who wore head coverings were the ones from Russia.

Rav Schechter immigrated to the United States in 1902 and became president of the JTS.

Rav Schochet immigrated to the United States in 1906.  In the photograph it looks like he's wearing a Russian fur cap, but this probably has more to do with the fact that he was from Russia and not Europe.

Rav Kohler immigrated to the United States in 1869 and helped build the foundation for Reform Judaism.  It is not surprising that he does not wear a kippah; what is surprising is that none of this more conservative colleagues did either.  This is often held up as one of the things that made Reform Judaism different, but I don't see much evidence for Orthodox or Conservative Jews wearing kippot in 1860's America.

A Conservative Jew who opposed Reform Judaism, he nevertheless did not wear a kippah.  He immigrated to the United States in the 1880's.

Rav Szold immigrated to the United States in 1859.

Rav Morais immigrated to the United States in 1851, and helped found the Jewish Theological Seminary in New York, immediately after the American Civil War.  A major influence on Conservative Judaism, he did not wear a kippah.

Rav Rice immigrated to the United States in 1840 and became the first ordained rabbi to serve in a rabbinical position in the US.  Although the small black velvet kippah was still common in German in 1840, Rav Rice did not wear one.