Two Byte Arrays - One Is Temp Buffer And Other Append Bytes Together

The relevant code below is not achieving what I hoped it would. I intend for
1. SerialPortBufferTemp to be used as the container for each Serial Port Read.
2. _buffer should contain the resultant array from two separate Reads, the second appended directly after the first.

(E.g. 60 bytes read on the 2nd Read will be appended to the 168 bytes received on the first Read (228 total resultant byte length))
The problem with my code below is that _buffer has increase in size from 300 bytes to 600 bytes, skipped bytes 168 to 300 (leaving them all zeros) and added the remaining 60 bytes from the second read beginning at _buffer[300].

private List<byte> _buffer = new List<byte>();
byte[] SerialPortBufferTemp = new byte[300];
serialPort1.Read(SerialPortBufferTemp, 0, iBytesRead);
if (iValidRecordCtr == 0) {
_buffer.Clear();
[Code] .....