Why are the other sons of Shuselach not listed?
Rashi: It seems that Eiran 1 had many descendents who were therefore called after him, whereas Shuselach's other children, who had smaller families, were called after Shuselach. 2
Malbim (26:36): The Pasuk in Divrei ha'Yamim (1:7:21) states that the Anshei Gas killed them (they left Egypt four hundred years after Bris Bein ha'Besarim, 30 years too early). Shuselach had another son named Beri'ah (1:7:23), for there was Ra'ah in his house; he is Eran mentioned here. His name switched from Ra to Er. He was called Eran to distinguish him from Er, Yehudah's Bechor.
What is the significance of the total number of families listed here?
Rashi (citing R. Moshe ha'Darshan): There are fifty-seven from Yisrael and eight from Levi. 1
The ''Hey in "Ki Atem ha'Me'at" Devarim 7:6 alludes to the five less than the (seventy) nations of the world (and "Yatzev Gevulos Amim le'Mispar B'nei Yisrael", in Devarim 32:8, implies that the numbers should be the same. PF) According to Sifsei Chachamim (refer to 26:16:1:2), Rashi himself holds that exactly five families died out. Refer also to 26:13:2:1*, where he explains why. See also Ramban's final comment on Pasuk 58.
