Kosen judo (高專柔道 Kōsen jūdō) was a refinement of Kodokan judo that was developed and flourished at the Kōtō senmon gakkō(高等専門学校) (Kōsen (高專)) technical colleges in Japan in the first half of the twentieth century. Kosen judo’s competition rules allowed for greater emphasis of ne-waza (寝技, ground techniques) than in mainstream judo and it is sometimes regarded as a distinct style of judo. Today, the term “Kosen judo” is frequently used to refer to the competition ruleset associated with it that allows for extended ne-waza. Such competition rules are still used in the shichitei jūdō / nanatei jūdō (七帝柔道 seven imperials judo)competitions held annually between the seven former Imperial universities.
(from the wikipedia article on Kosen Judo)
By the way, Kosen Judo is not a lost art, because people still practice it. 7 national universities in Japan (i.e. Kyoto, Tokyo, Nagoya, Hokkaido, Kyushu, Osaka, and Tohoku universities) have Judo clubs that adopt this style of Judo, and hold annual tournaments among each club.
Many BJJ players wrongly believe that techniques such as X guard, De La Riva guard or Spider guard are BJJ specific moves, but they have been used in Judo (especially Kosen Judo) for years even before Marcelo Garcia, Ricardo de La Riva etc.. brought it to the attention of the world (since the 90’s). Just like many techniques that seem to be have been recently invited, we can never be sure that other martial artists in the past weren’t doing the exact same move.
Here is a Kosen Judo tournament in 1952. You can see use of Spider Guard, De La Riva Guard, some Z Guard:
In this video we can see the judoka of U.Nagoya of Kosen Judo, demonstrate a position that looks like X guard from BJJ, under the supervision of Shihan Mitsunosuke Kosaka. However that exists in Nanatei-Judo and is similar to the BJJ guard, as we see in the last two videos:
2nd Video: Tournament Nanatei-Judo 1988
3rd Video: Tournament with rules of Nanatei-Judo 2008.