284 The American Naturalist. [March, 
Systrurus Garman, 1883. 
Lateral expansion of the petrosal region slight. Squamosal short 
and its posterior end widened into a hook-like process for the attach- 
ment of the quadrate. Pterygoids not toothed posterior to their junc- 
tion with the ectopterygoids and their posterior expanded portions more 
curved than in other genera. The ectopterygoids are grooved for the 
reception of the pterygoids. However, their posterior ends are not 
notched, as in Ancistrodon, but are attached to the pterygoids for a 
much greater length than in either of the remaining genera. The pal- 
atines are triangular and attached to the pterygoids at the acute angle, 
the obtuse angle being nearer the anterior end of the palatines. Post- 
frontals rudimentary. Posterior caudal vertebre céossified, i.e., end- 
ossicle present. 
Number of vertebree of the body from 130 to 153; caudal vertebra 
from 20 to 35. 
United States and Mexico. 
I omit the specific characters until opportunity is offered for the 
study of a wider range of specimens. 
Crotalus Linne, 1758. 
Petrosal region expanded and the outer edge of the petrosal slightly 
upturned so as to form a support for the squamosal. The parietals 
possess clearly marked lateral expansions which connect with the an- 
terior ends of the petrosals. Squamosals relatively more curved than 
in other genera, not hooked, but their posterior ends expanded. Ptery- 
goids not toothed posterior to their junction with the ectopterygoids.* 
Posterior ends of the ectopterygoids grooved but not notched, and the 
pterygoids not notched for the reception of the ectopterygoids. The 
palatines are club-shaped and attached to the pterygoids by their more 
pointed ends. The postfrontals are well-developed, border the orbital 
surface of the parietals, and connect with the frontals. The posterior 
caudal vertebre are coossified, i. e., possess a well-developed end-ossicle. 
Number of vertebre of the body from 165 to 187; tail from 19 to 32. 
United States, Mexico and Brazil. 
EXPLANATIONS OF PLATES. 
Fig. 1—Squamosal of A. piscivorus. 
Fig. 2.—Squamosal of A. contortriz. > 
Fig. 3.—Squamosal of S. catenatus. 
° If Dumeril's plates be correct, C. durissus does not agree in this respect : see 
Prodrome de Classification des Ophidiens, 1852. Planche 2, Figs. 14 and 15. 
