Mini ethernet shield pins don't have SI
The "Mini Ethernet Shield" uses ENC26j60 chip. The Arduino Ethernet library is for Wiznet W5100/W5200/W5500 chips. You can't use it with ENC26j80.

With Arduino Mega I would recommend to use the UIPthernet library for ENC26j80. Other option is the EtherCard library.

The UIPEthernet library has the same API as the the Arduino Ethernet library.