ON THE SENSORY PIT OF THE CROTALIN^. 51 



All the above were spirit speciineuSj and only a few were 

 in a sufficient state of preservation to admit of any detailed 

 histological investigation. 



I. The Structure of the Organ in the Adult 

 Animal. 



The precise position of the external opening of the 

 sensory pit is somewhat variable; it is situated either in a 

 direct line between the eye and the nostril, or somewhat 

 below this line. In some species of Cro talus it is almost 

 immediately underneath the nostril, ©• g- Crotalus terri- 

 ficus (cf. PI. 4>, fig. 4). Its form is generally oval or pear- 

 shaped, with the narrower end directed posterioi-ly. Fi'om 

 this posterior extremity a groove passes in a slightly upward 

 direction to the eye. This feature is more marked in some 

 species than in others, and among the snakes examined it 

 was most conspicuous in Lachesis mutus, L. atrox, L. 

 lanceolatus, and Crotalus terrificus. In the first- 

 mentioned animal the groove was partly protected by a 

 distinct flap of tissue, which on being lifted up exposed 

 more of the cavity of the pit (PI. 4, fig. 2). In Lachesis 

 lanceolatus prolongations of the scales forming the an- 

 terior and inferior borders of the pit enter respectively into 

 the construction of the more external portions of the anterior 

 and inferior walls (PL 4, fig. 12). 



The external orifice leads into a chamber, the inner wall of 

 which is very thin and membranous. This membranous wall 

 is a partition separating the outer chamber from a second 

 more deeply situated inner chamber. 



The outer chamber (PL 4, fig. 14, o. c.) is lined by a 

 smooth cuticle continuous with that of the rest of the head. 

 This cuticle is from 3"5 to 5 ju in thickness, and in adult speci- 

 mens (after preservation) it easily comes away from the thin 

 wall of sensory tissue underlying it. At its junction with 

 the cuticle of the scales bordering the orifice there is a con- 

 voluted depression, lined with crenulated cuticle of the same 



