MEXICAN HERPETOLOGICAL MISCELLANY—SMITH 355 
keeled and mucronate, with a number of fine, lateral mucrones; scales 
between lateral nuchal pocket and foreleg keeled, rather strongly 
mucronate, the largest larger than scales posterior to ear or in tem- 
poral region. 
Dorsal scales keeled, mucronate and denticulate to a moderate de- 
gree, 31 from occiput to base of tail; 36 scales around middle of body; 
13 to 14 femoral pores; 22 lamellae under fourth toe; snout to 
vent 77 mm.; tail 112 mm.; snout to posterior margin of ear 19.9 mm.; 
snout to occiput 15.9 mm.; fourth toe 21.7 mm.; hind leg 55.5 mm. 
Color—Dorsal surface yellowish gray (slightly bluish where scales 
are shed) ; a narrow, black nuchal collar, complete on dorsal surface, 
scarcely visible on throat; collar not extending onto arm, not over 
two scales in width. Dorsal surface otherwise unmarked. Gular 
region and chin uniform blue, except a gray area about throat in 
front of chest; latter white; sides of abdomen apparently lavender, 
with a broad berder covering a width of three scales; latter extend- 
ing from groin nearly to a line even with axilla, but not extending 
laterally anteriorly to enter axilla. Ventral surfaces of limbs and tail 
white. 
Remarks —This specimen was referred by me in 1936° to serrifer, 
although several of its peculiarities were pointed out; and in 1939 
it was referred to mucronatus omiltemanus. Recent material from 
Chiapas and Guatemala has shown that the characters previously 
considered as anomalous have considerable significance. This speci- 
men does not belong to serrifer, although the belly pattern is the 
same, since it has the supraoculars in two rows and the frontonasals 
separated from each other. The latter species is the nearest one of the 
poinsettii group; the more remote omiltemanus and its relatives, al- 
though having similar supraoculars, have the frontonasals in contact 
and a different belly pattern. In fact, it is believed that prezygus 
belongs to the formosus rather than to the poinsettii group, since it 
has a very narrow collar not light bordered (apparently). It differs 
widely from others of the formosus group, however, in having large 
dorsals, the frontoparietals separated from each other, supraoculars 
in two rows, etc. It appears to be a link between the formosus and 
poinsettii groups, in the same manner as lunaei and acanthinus are 
obviously links between the formosus and spinosus groups. All mem- 
bers of the latter group can be traced to acanthinus and lunaei, while 
all of the poinsettii group can be traced to prezygus and serrifer. The 
name prezygus refers to the phylogenetic position of this species, near 
the ancestral type of the collared (pocnsettiz) group of the genus. 
5 Kansas Univ. Sci. Bull., vol. 25, pp. 561, 562, 1936. 
® Field Mus. Nat. Hist., zool. ser., vol. 26, p. 221, 1939. 
