'getSalinitySensorPeriod' was not declared in this scope..how to solve that?

//constant that won't change 
const int salinitySensorPin = 2 ; //salinity sensor pin 

//setup fucntion that run at very beginning of the program

void setup() 
  Serial.begin(9600); //set data rate to 9600 bps
  pinMode (salinitySensorPin, INPUT); 


void loop() 
  //cheack the salinity period and calculate salinity frequency
  float salinityPeriod = getSalinitySensorPeriod (); //assign RETURN value 
  float salinityFrequancy= 1000 / salinityPeriod ; // to calculate the frequency

   //display period and frequency
   Serial.print("salinityPeriod = ") ; //display salinity period 
   Serial.print(salinityPeriod); //display measured period 
   Serial.println(" us"); //microsecond unit 
   Serial.print("salinityFrequency = ");//display salinity frequency
   Serial.print (salinityFrequency,3);//display neasure frequency 
   Serial.println(" khz");//unit heartz

   //apply treadline to convert the percent salinity then display salinity
   float salinityValuePercent = 0.01 * salinityFrequency + 0.05;//salinity percent 
   Serial.print("Salinity (before calibrated)= %";//salinity = '
   Serial.print(salinityValuePercent,2);//display measured in percent mass
   Serial.println(" %");//display percent 
   delay(2000);// 1 second delay in read 
//a function for retrieving the salinity sensor reading 
//in a period of microseconds
int getSalinitysensorPeriod()
  int timeStart = 0 ;
  int timeStop = 0 ;
  //wait for pin to go low 
  while( LOW ! = digitalRead(salinitySensorPin)){}
  //wait for the pin to go high and reacord the time
  while (HIGH ! = digitalRead(salinitySensorPin){}
  timeStart = micros();
  //wait for the pin to go low a second time 
  while (LOW ! = digitalRead(salinitySensorPin)){}
  //wait for the pin to go high and record the time 
  while(HIGH ! = digitalRead(salinitySensorpin)){}
  timeStop = micros();

    //return in period which the diffrent between 
    return timeStop - timeStart;

getSalinitySensorPeriod () != getSalinitysensorPeriod()

C is case sensitive.