64 WATER REPTILES OF THE PAST AND PRESENT 



the surface to breathe only, the nostrils are situated so that they 

 are the first to emerge, that is, they are near the front end. The 

 crocodiles, with a more or less elongated face, as also the Choristo- 

 dera, described farther on, are exceptions, since their nostrils are 

 at the extremity of the snout. Both of these tj^es, however, not- 

 withstanding the elongation of the face, are only partly aquatic in 

 habit, and in the crocodiles the breathing organs have undergone 

 a strange modification in accordance with habits pecuharjy their 

 own, as will be explained later on. Whether this recession of the 

 nostril toward the eyes can be explained in all cases by the peculiar 

 breathing habits is, however, doubtful. Possibly in some cases, 

 such as the phytosaurs, described later, the creatures used their 

 long beaks to probe in the mud while breathing. Possibly the 

 posterior position has been in some cases rather the result of the 

 elongation of the face, leaving the nostrils behind in some forms, 

 or carrying them forward in others. Nevertheless posterior nos- 

 trils always indicate more or less aquatic habits. 



In all the earHest reptiles, as we have seen, the neck was short, 

 like that of their immediate progenitors, the ancient amphibians. 

 The shoulders were close to the skull, with not more than two verte- 

 brae that could be called cervical. It happens that most of the 

 earliest reptiles, as we know them, were more or less amphibious in 

 habit, and all of them were probably good swimmers; nevertheless 

 in all UkeKhood reptiles began their career as a class with a very 

 short neck. The earliest known distinctly terrestrial reptiles had 

 a moderately long neck composed of six or seven cervical vertebrae. 

 It may therefore be assumed with much probabihty that all later 

 reptiles with a greater or less number of cervical vertebrae are 

 speciaHzed animals, so far as the neck is concerned. Most living 

 reptiles have eight cervical vertebrae; a few have nine, and still 

 fewer have but five. Birds may have as many as twenty-four, 

 while all mammals, with two or three exceptions, have the primitive 

 number seven. Among extinct reptiles, however, there were not 

 a few with more numerous neck vertebrae, some having the enor- 

 mous number of seventy-six. 



An ordinary fish has apparently no neck whatever, the trunk 

 being seemingly attached to the head, nearly as in the primitive 



