1106 REPORT OF NATIONAL MUSEUM, 1898. 



every third or fourth gastrostege there is a small black spot through- 

 out the length to the tail. There is a larger black spot on the sides 

 between the extremities of a few of the cross bands. The superior bor- 

 der of the sixth and the adjacent part of the fifth superior labial is 

 black. On the top of the head are three large round black spots; one 

 is on the center of the frontal and one is on the anterior part of each 

 parietal. No cross bands on the muzzle. The dark cross bands are 

 only two scales wide on the posterior part of the body; on the anterior 

 part they are three or four scales wide. The interspaces vary from 

 twelve anteriorly to seven posteriorly. 



One specimen, Cat. No. 14268, was sent from near the city of Chihua- 

 hua by Mr. Edward Wilkinson to the U. S. National Museum. This 

 species is nearest the T. hiscntatus Duraeril and Bibron in squamation, 

 but differs greatly in coloration from this or any other species of the 

 genus. 



SIBON Fitzinger. 



Sihon Fitzinger, Neue Class. Reptilieu, 1826. p. 29. 



Heterurus Dumii;ril and Bibron, Erp. Gou., VII, 1854, p. 1170. 



Leptodira GCnther, Cat. Col. Snakes Brit. Mus., 1858, p. 165 (from nomen nudum 



of Fitzinger, 1843). 

 Eteirodipsas Jan, ElencoSist. Ofidi, 1863, p. 105. 



An elongate grooved tooth on the posterior part of the maxillary 

 bone; other teeth subequal. Head plates normal ; one loreal. Preaual 

 and subcaudal scuta double; scales smooth, with two apical pits. 

 Pupil vertical. Hemipeuis undivided with bifurcate sulcus spermaticus, 

 and numerous spines below and calyces above. The latter present a 

 free margin to the superior spinous region, that is, the organ is capitate. 



This genus has near allies among the Dipsadine group, to which it 

 belongs. From Dipsas it is distinguished by the divided preanal plate; 

 from Himantodes by the double scale-pits, that genus having but one. 

 From Trimorphodon it differs in the equality of the ungrooved maxil- 

 lary teeth and the single loreal plate. The greater number of species 

 of this genus are Mexican and Central American, one species {S. annu- 

 latum) extending its range throughout tropical South America. One 

 species only has been found on the Rio Grande River, and extends 

 within our borders. The species are closely allied and are subject to 

 some variation. One only (;S^. rhombiferum Giinther) I have not seen, 

 and I give its characters on the authority of Oiiuther. All the spe- 

 cies known have but one temporal plate in the first row. 



In some of the forms the head is less distinct j)osteriorly than is 

 cbaracteristic of the typical genera of this subfamily, giving a coronel- 

 line form and forming a transition to the more fusiform types. 



The 8. annulatum is the only species which ranges over Brazil ; the 

 others are all Central American ( = Mexican). 



