I am following Klauber (1952) in assigning some of the specimens of this species from Michoacán to the subspecies aquilus and others to C. t. triseriatus. The distinguishing characters of these subspecies are given by Klauber (1952:28). On the basis of the few localities from which the species is known in Michoacán it seems as though C. t. aquilus inhabits the open grassy areas on the Mexican Plateau and the associated open pine-oak or oak-bunch grass habitats to the north and east of the Cordillera Volcánica. Crotalus triseriatus aquilus has been collected at elevations from 1600 to 2000 meters in Michoacán.
Crotalus triseriatus triseriatus (Wagler)
Uropsophus triseriatus Wagler, Natürliches System der Amphibien, p. 176, 1830.—México. (Probably Mexico City.)
Crotalus triseriatus triseriatus, Klauber, Bull. Zool. Soc. San Diego, 26:19, August 8, 1952.
Cerro Tancítaro (36); Opopeo; Pátzcuaro.
This small rattlesnake inhabits rocky areas in pine and pine-oak forests above 1600 meters in the Cordillera Volcánica; it has been collected at 3270 meters on Cerro Tancítaro. The series reported by Schmidt and Shannon (1947:84) is a mixture of specimens of Crotalus triseriatus and Crotalus pusillus. The two species are found together on Cerro Tancítaro, but only Crotalus pusillus inhabits the coniferous forests of the Sierra de Coalcomán. Klauber (1952:30) stated that despite the proximity of Crotalus triseriatus triseriatus and Crotalus triseriatus aquilus in Michoacán, there is no evidence of intergradation. He went on to suggest that additional material might show that the two named populations actually are distinct species. The specimens that have been studied since Klauber's investigations also show no evidence of intergradation, but there still is no known sympatry of the populations.
The small montane rattlesnakes belonging to the species C. pricei, C. pusillus, and C. triseriatus present a problem in systematics and distribution worthy of intensive investigation. A knowledge of the distribution and relationships of the various populations of these snakes, together with other species also living in isolated populations on the higher mountains in México, probably will be of great significance in understanding dispersal and differentiation of animals during the Pleistocene.
SPECIES OF QUESTIONABLE OCCURRENCE
Some species for which there are no authentic records from Michoacán can be expected there on zoogeographic probability. Other species have been recorded from Michoacán, but these records are doubtful for any one of several reasons. Fifteen species of such questionable occurrence are discussed below: