536 



EIGENMANN 



Fig. 22. Side view of head of 

 Rhitieura showing surface plates 

 and position of eje in relation to 

 them. 



of the head is continuously covered with plates. There are four 

 labials (i, 2, 3 and 4, of fig. 22), the posterior of which is compara- 

 tively large. Above the labials from in front backward lie a single 

 nasal (5), a single loreal (6), a single preorbital (7), and a group of 

 temporals (8). Above this series of plates lie a supranasal (9), joined 

 to its fellow of the other side, a prefrontal (11) and two supraciliaries 

 (12, 13). In heads cleared with xylol the black eye can be seen to 



lie underneath the angle between the 

 two supraciliaries and the preorbital. 

 The dermis and epidermis over the 

 eye are not different from these struc- 

 tures over neighboring regions ex- 

 cept that in one instance (PL xxxii, 

 fig. 3, dt') a solid column of cells 

 32 /x thick extends from Harder's 

 gland to near the epidermis, with- 

 out however fusing with the latter. 

 Fisher (1900, p. 470) found that in 

 Trogonophis the epidermis is re- 

 duced to half its thickness and free 

 from pigment over the eye. In Amphisbcena strauchi and A. dar- 

 ivini the skin is not thinner and the pigment is little or not at all less 

 over the eye. A conjunctival sac has been described for various 

 Amphisbaenians. No such structure is present in Rhineura. 



Harder's gland (PI. xxxii, figs. 2 and 3, UGl.) is out of all propor- 

 tion to the size of the eye. In a horizontal section it measures about 

 four times as long as the eye (medio-laterally) and three times as wide 

 (antero-posteriorly) . Duvernoy found that in Typhlops Harder's gland 

 is ten times as great as the eye. It is divided into two distinct lobes, 

 that over the anterior face of the eye is histologically quite different 

 from that over the posterior face. In vertical section the gland is seen 

 to entirely surround the eye except sometimes at its lower posterior 

 quarter. The large size of Hai'der's gland has given rise (Duvernoy) 

 to the conclusion that its function is not connected with the eye. Its 

 secretion is poured directly into the tear duct and through it into the 

 nasal cavity. 



The distance of the eye beneath the outer surface of the epidermis 

 measures between 320 and 560 microns in specimens between 280 and 

 310 mm. long. It is surrounded by two layers of connective tissue. 

 These are thin over the distal half of the eye. Over the proximal 

 narrow end of the eye they become thick and since they are prolonged 



