Group D 



1914.] NATURAL SCIENCES OF PHILADELPHIA. 67 



The extremes in structural variation in the form of the cerci are a 

 simple incurved falciform type, found in the group A, and one with 

 a median tooth and an accessory lobe from the base, found in group 

 F. The general cereal structure of the various groups can be pre- 

 sented best in tabular form. 



[ mexicana 1 

 Group A j falcata I ^ii^^P^G) falciform. 



Group B (serrifera). — Simple, dorsal margin serrato-dentate. 



Group C {polUcifera). — With an external median tooth. 



Incertse sedis (diirangensis) . — With evidence of a dorsal median 



tooth. 



[ castanea ; 



brevihastata With a dorsal median fork, developing 



gladiator from a simple tooth to a large flattened 



emarginata | lobe covering the greater portion of the 



oreoeca distal section of the cereal shaft. 



catinata 



Group E {fauriformis) . — With a greatly developed lobiform median 

 tooth arising from the external margin of the shaft, the distal portion 

 of the latter peculiarly modified. A transverse proximal lamella 

 present dorsad on the shaft. 



I tridactyla 1 With a dorsal median tooth and an accessory 

 Group F I caMdelli J digitiform lobe from the base of the shaft. 



The species in group D exhibit a regular development in the form 

 of the dorsal median fork from a simple median tooth which becomes 

 depressed and flattened, spreading laterad until it is as wide as the 

 proximal portion of the shaft, to the other extreme which has it 

 modified into a great inverted spoon-like plate covering the greater 

 portion of the cereal shaft. The peculiar digitiform accessory 

 appendage of tridactyla and caudelli springs from a proximal trans- 

 verse ridge, which is apparently homologous with the more decided 

 transverse lamella found in the same region in tauriformis. 



From the evidence of eighteen immature males, 'belonging to six 

 species {durangensis, brevihastata, gladiator, oreoeca, catinata, and 

 polUcifera), it is evident that the separation of the median fork of 

 the cercus is never accomplished before the mature condition. Of 

 brevihastata and polUcifera we have material representing two con- 

 secutive instars, one preceding the mature condition, the other 

 species being represented by this stage alone. In the forms of 

 which we have two stages no indication of the lobe is apparent in 



