SIBYNON XEBULATUS. 143 



OBSERV. The synonyms not quoted above, may easily be found in 

 the systematic writers on the subject. Had we had more ample mate- 

 rials at our command, we would have investigated more thoroughly 

 the history of the present species, represented in the collection of 

 the United States Exploring Expedition by a single specimen, yet 

 immature. 



DESCR. The head is depressed, superiorly flattened, ovoid when 

 viewed from above, and quite distinct from the neck. The snout is 

 rounded and the jaws even. The cleft of the mouth is large, sub- 

 concave beneath the eye, and not raised upwards towards its angle. 

 The eyes are well developed and circular, and the nostrils situated 

 between two plates. The occipital plates are large, admitting the 

 posterior extremity of the vertex plate between their commissure. 

 The vertex plate is broad and short. The supraoculars are nar- 

 rower anteriorly than posteriorly, and nearly as long as the vertex 

 plate, though extending a little more forwards. The postfrontals 

 are quite large, subangular, extending somewhat to the loral region. 

 The prefrontals are very small, subtriangular, and declivous forwards. 

 The rostral is subcouical, concave inferiorly, occupying exclusively the 

 anterior portion of the snout. The prenasal is larger than the post- 

 nasal; the nostril apparently perforates equally both plates. There 

 is no loral. A large and subquadrangular anteorbital is contiguous 

 to the postnasal, being a little longer than deep. We observe two 

 postorbitals on the left and three on the right, but the inferior two 

 are very small compared to the upper one, so in fact the normal num- 

 ber is probably two, the uppermost being, at any rate, the largest. 

 The temporal shields are scale-like, and a little larger than the scales 

 themselves. There are six upper labials, the third and fourth enter- 

 ing into the orbit ; the fifth and sixth are the largest, horizontally 

 elongated, whilst the anterior four are deeper than long, and nearly 

 equal in size, with the exception of the fourth, which is a little larger 

 than the anterior three. The symphyseal is small and subtriangular, 

 entirely inclosed by the first pair of lower labials, which are well 

 developed. There are ten pairs of lower labials; the second, third, 

 and fourth are smaller than the first, and slightly increasing in size 

 backwards ; the fifth is the largest ; the posterior five are irregular 

 in size, and elongated horizontally. Three pairs of subquadrangular 



